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Fundamental Diagram and Stability of Mixed Traffic Flow Considering Platoon Size and Intensity of Connected Automated Vehicles

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Abstract

The connected automated vehicle platoon is a critical organizational form of the intelligent transportation system in the future. Considering the platoon size and intensity of connected automated vehicles (CAVs), this paper studies the fundamental diagram and stability of mixed traffic flow. Firstly, the spatial distribution of vehicles in the mixed traffic flow is analyzed. Based on the platoon intensity and Markov chain, the proportion of different types of vehicles in the mixed traffic flow is deduced. Then, the general model framework of the fundamental diagram of the mixed traffic flow is established. The influence factors of the fundamental diagram under the critical scenarios are discussed. Finally, the general condition of mixed traffic flow stability considering the platoon size and intensity of CAVs is developed. Numerical experiments are adopted to analyze the influence of the characteristics of the CAVs platoon on stability. The results show that: (1) a greater platoon size leads to the increase of traffic capacity while it is harmful to the maintenance of traffic flow stability; (2) the platoon size is recommended to be set at 4 to 6 to balance the relationship between traffic capacity and stability; (3) a more significant platoon intensity can help improve the traffic capacity and stability; (4) the penetration rate of CAVs has a positive effect on the traffic flow stability until it increases to a certain degree.
Preprint submited to Physica A: Statistical Mechanics and its Applications

Fundamental diagram and stability of mixed traffic flow considering platoon size and intensity of
connected automated vehicles







Abstract
  
     

                
                

             



       
     


Keywords: Mixed traffic flow; Platoon size; Platoon intensity; Fundamental diagram; Stability
to: Yangsheng Jiang, School of Transportation and Logistics, Southwest Jiaotong University, Chengdu, Sichuan
610031, China, E-mail:
1
1. Introduction



[35]

             





[912][1316]
[1719]
[2022]      
               
              

[2628]

                
       








              

              
               




    


  




  

2
2. Literature review
2.1. 
          




               










2.2. 

                








                  

 



2.3. 
             
              
                 


        

               

        
3
               





              


[4951]

        

                 




              

                


 [2022,54,55]
              

            

             
          


       















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

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
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

4









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



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This paper

3. Mixed traffic flow model
3.1. 
3.1.1 Spatial distribution of vehicles







 

 
 




 


 
Maximum platoon size:
    
H C
Platoon size:
HH LC FC DC
H
5
             
 

 

 

 
3.1.2 Platoon intensity

                  


 
     

 
 


 





   󰇝󰇞

󰇟󰇠

  󰇛󰇜 
H C
(a) Mixed state 
Platoon size:
Platoon size:
Platoon size:
(b) Random state 
(c) Separated state 
6
 󰇛󰇜

󰇣
󰇤

             󰇛󰇜







󰇛󰇜󰇛󰇜󰇱󰇛󰇜



󰇛󰇜󰇛󰇜󰇛󰇜

󰇛󰇜󰇛󰇜󰇱󰇛󰇜



󰇛󰇜󰇛󰇜󰇛󰇜

3.2. 
              
 
  




                 

 󰇛󰇜󰇛󰇜󰇛󰇜

H C
Platoon  -- Vehicle Platoon  -- Vehicle 
HH LC FC DC
H Maximum platoon size: Platoon size:
7

 󰇛 󰇜󰇛󰇜󰇛󰇜



 󰇝  󰇞



 󰇝  󰇞
󰇝󰇞
󰇝󰇞󰇝 󰇞
󰇝󰇞
󰇝󰇞

 󰇝󰇞 

 

 󰇝 󰇞
󰇝󰇞

  󰇝󰇞 
 
 

 
󰇝󰇞



 󰇛 󰇜󰇛󰇜󰇛󰇜



󰇝󰇞



󰇝󰇞





8
  







Lemma 1. 󰇝󰇞
Proof. 
Scenario A:             󰇛󰇜 

󰇝󰇞󰇝󰇞


  
Scenario B:   
󰇛󰇜󰇛󰇜󰇛󰇜󰇛󰇜

  
󰇝󰇞󰇝󰇞
󰇛󰇜


Scenario C:   ,
󰇝󰇞
󰇝󰇞
 




 


  
  
 

 󰇛󰇜
󰇛󰇜

 

 󰇟󰇡󰇥
󰇦󰇢󰇠
󰇡󰇥
󰇦󰇢
󰇟󰇛
󰇜󰇠
󰇛󰇜
󰇟󰇛󰇜󰇠
󰇡
󰇢


9
3.3. 
3.3.1 Human-driven vehicles mode

              
       

󰇛󰇜󰇧󰇛󰇜
󰇨󰇧
󰇛󰇜󰇛󰇜
󰇛󰇜󰇨
󰇛󰇜󰇛󰇜󰇛󰇜󰇛󰇜󰇛󰇜


 󰇛󰇜    󰇛󰇜    
 
 󰇛󰇜 
  󰇛󰇜󰇛󰇜󰇛󰇜 󰇛󰇜    󰇛󰇜
󰇛󰇜󰇛󰇜 󰇛󰇜     
           
  
  
3.3.2 Gap-regulating mode

       

󰇛󰇜󰇛󰇜󰇛󰇜󰇗󰇛󰇜
󰇛󰇜󰇛󰇜󰇛󰇜󰇛󰇜

 󰇛󰇜   
  
  
  
            

󰇛󰇜󰇡󰇛󰇜󰇛󰇜󰇢󰇛󰇜


3.3.3 Cooperative adaptive cruise control mode

              
              

󰇫󰇛󰇜󰇛󰇜󰇛󰇜󰇗󰇛󰇜
󰇛󰇜󰇛󰇜󰇛󰇜󰇛󰇜

10
  
              
 
 

󰇛󰇜󰇡󰇛󰇜󰇛󰇜󰇢󰇛󰇜


3.3.4 Adaptive cruise control mode
               


󰇛󰇜󰇛󰇜󰇛󰇜󰇛󰇜󰇛󰇜

                  
   
4. Fundamental diagram
4.1. 
               
              
             
                  
                


 󰇛󰇜  󰇛󰇜 






󰇛󰇜
󰇛󰇛󰇜
󰇜

󰇛󰇜
󰇛󰇛󰇜
󰇜

󰇛󰇜

󰇛󰇜

󰇛󰇜
4.2. 
             
        
             

11


 
            



 





 


    



󰇛󰇜


 




󰇛󰇜

  




󰇛
󰇜
 




Lemma 2.   

Proof.   
12
                
  
Scenario C:   


    

      




     
  
 
 󰇛󰇜
  
󰆒 

󰆒
 󰇛󰇜

󰇛
󰇜

  
󰆒󰆒 

󰆒󰆒
󰇛
󰇜


 


4.3. 























4.3.1 Scenario A: when



 
 

13
 
4.3.2 Scenario B: when 
 
  

             

 



  
4.3.3 Scenario C: when and 
4.3.3.1. Maximum platoon size
   
 
 
 
            
14
 
(a) 
(b) 
(c) 
(d) 
(e) 
(f) Capacity

   
15
          
 

 
                  
       

             

  

  


           
 
































































16

4.3.3.2. Platoon intensity

    
                
             

              

(a) 
(b) 
17
(c) 
(d) 
(e) 
(f) Capacity
 
        

  
 


 

   


           
 


 

18


























































































         

   
      
   
            
 
 
4.3.3.3. The penetration rate of CAVs

  
19



(c)
(d)
(e) 
(f) Capacity

(a)
(b)
20
(c)
(d)
(e) 
(f) Capacity

(c)
(d)
21
(e) 
(f) Capacity
   





 




    and 

























































5. Stability analysis
5.1. 

              



22

󰇛󰇜




                
 󰇛󰇜
 




 󰇩󰇛
󰇜



󰇪󰇡
󰇢
           
    
    


5.2. 
5.2.1 Scenario A: when
                
  





 
5.2.2 Scenario B:when
 
 
 




 

 

                


23
 

  





(a)
(b)
(c) 


              
 

24

5.2.3 when and 

  
  



󰇛
󰇜
 




 
5.2.3.1. Maximum platoon size
    
               
 





               

              

25


 
5.2.3.2. Platoon intensity

   




                
          
                 

  

 

  
5.2.3.3. The penetration rate of CAVs
             
   
26




 
 
 


 
6. Conclusions and Future Work
              
         

                 
 
                
      

  
               
            
 

                 
 



 

   
                
 
 
27

                 
 
 

                


    
               
 

                 



Declaration of Competing Interest





CRediT authorship contribution statement
Zhihong Yao: 
 Qiufan Gu:        
 Yangsheng Jiang:         
Bin Ran: 
Acknowledgments


          

References
 

     
 
        

 

 

 

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
 

             

 

 
         

       
            

 


 
          

 
            
            

 
           

     
            

 
             

 
       

 

      
          

     

 

              

                


29
  

 
           

             
         

 
             

 
  

 
             

 

              


 

                  
            

    


 

 
         

               
            

 


 

 


 

 
30

      


 

 


       
       

 
           

             
           

 


  
            

 
            

                 

 
         

 

                  


               
          

  

             
             

             
           

               
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            

 

             


        
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
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               
         
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... While the influence of factors such as platoon intensity, platoon size, and the stochasticity of time headways on the mixed traffic flow has been carefully and deeply investigated in previous studies (T. Chen et al., 2023;Ghiasi et al., 2017;Yao et al., 2023;Yao, Gu, et al., 2022;J. Zhou & Zhu, 2020, especially by Zhu (2020, 2021) and T. Chen et al. (2023). ...
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This paper proposes a fundamental diagram of the mixed traffic flow for multi-lane road segments with connected automated vehicles (CAVs) dedicated lanes based on a flexible and efficient lane management strategy for CAVs. First, two lane management strategies of CAVs dedicated-HDVs lane strategy (i.e., C-H strategy) and CAVs dedicated-Mixed lane strategy (i.e., CM strategy) are introduced. The more efficient CM strategy is chosen based on the characteristics of the two strategies, and the five stable states under this strategy are analyzed. Second, different car-following modes in mixed traffic flows are analyzed, and general fundamental diagrams for homogeneous and mixed traffic flows are derived based on car-following models. Then, the stabilized traffic conditions are derived based on the traffic flow characteristics in the five stable states. A fundamental diagram model for the multi-lane road segment with CAVs dedicated lanes is developed based on the general fundamental diagram, and some properties of the fundamental diagram are derived. Finally, the impacts of different numbers of dedicated lanes and traffic conditions on the traffic volume and capacity are analyzed based on the fundamental diagram, and the optimal settings of dedicated lanes for CAVs under different lane number scenarios are given. Numerical simulation results showed that: (1) there are five stable states for the traffic flow on the road segment under the CM strategy; (2) properly setting up CAVs dedicated lanes can effectively improve the efficiency of road segments; (3) under the CM strategy, for any CAVs dedicated lane setting scenario, the traffic capacity of the road segment shows a gradually increasing trend as the penetration rate of CAVs increases. (4) compared to the C-H strategy, the capacity of the road segment under the CM strategy will be higher than the C-H strategy when the CAVs penetration rate exceeds a certain threshold. Moreover, as CAVs penetration rate increases further, the advantages of the CM strategy in terms of traffic capacity become more and more significant.
... The stability of traffic flow primarily investigates the propagation patterns of stochastic disturbances among vehicles, constituting one of the fundamental characteristics of car-following behavior [28][29][30]. Based on the magnitude of disturbances, stability analysis can be categorized into nonlinear stability [31][32][33] and linear stability [34][35][36]. Nonlinear stability analysis focuses on studying the stability characteristics of traffic flow under the influence of significant disturbances. This often involves the application of perturbation methods to derive shockwave equations for microscopic or macroscopic traffic flow models [32]. ...
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With the development of autonomous driving and vehicle-to-infrastructure technologies, connected automated vehicles (CAVs) will gradually replace human-driven vehicles (HDVs). However, during the widespread adoption of CAVs, the roads will experience a new type of mixed traffic flow composed of vehicles with different automation levels for an extended period. To analyze the impact of different automation levels on mixed traffic flow, this paper proposes a stability analysis framework for mixed traffic flow that considers different automation levels. The influence of automation level classification on the composition of mixed traffic flow is analyzed first, and the degradation of CAVs in mixed traffic flow is further explored. Then, a stability analysis framework for mixed traffic flow considering different automation levels is proposed, and the impact of key parameters on the stability is analyzed. Finally, simulation experiments are designed to validate the theoretical analysis results. The research findings indicate that (1) the stability of mixed traffic flow improves with the increase in steady-state speed. When vehicle speed exceeds 20 m/s, mixed traffic flow remains stable. (2) When the CAV penetration rate is low, the stability of mixed traffic flow is poor due to the degradation of CAVs. However, with the increase in the penetration rate and automation level of CAVs, mixed traffic flow rapidly tends toward a stable state. (3) Numerous factors influence the stability of mixed traffic flow, with the order of importance being the CAV penetration rate, automation level, headway, and maximum acceleration. Therefore, this study can provide theoretical support for managing and controlling mixed traffic flow with different automation levels in the future.
... Te CNN part is followed by the LSTM part. LSTM, which is a variant of recurrent neural networks (RNN), incorporates input gates, output gates, forget gates, and memory cells to address the challenge of gradient vanishing or explosion [49][50][51]. Figure 7 shows the structure of LSTM unit. Te forget gate (FG) determines which information about the state of the cell is lost. ...
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Accurate vehicle acceleration prediction is useful for developing reliable Advanced Driving Assistance Systems (ADAS) and improving road safety. The existence of driver heterogeneity magnifies the variations in acceleration data, leading to consequential impacts on the precision of vehicle acceleration prediction. However, few studies have fully considered the driver heterogeneity when predicting vehicle acceleration. To model the characteristics of individual drivers, this study first identifies the driving behavior semantics which is defined as the underlying patterns of driving behaviors. The analysis results from the coupled hidden Markov model (CHMM) are used to evaluate the driving behavior differences between different drivers by Wasserstein distance. Then the convolutional neural network (CNN) and long short-term memory (LSTM) network are applied to predict vehicle acceleration. To validate the accuracy of the proposed prediction framework, vehicle acceleration data in car-following conditions is extracted from the safety pilot model deployment (SPMD) dataset. The segmentation results indicate that the CHMM possesses a robust capacity for modeling driving behavior. The prediction results demonstrate that the proposed framework, which incorporates driver clustering before prediction, significantly improves the accuracy of predictions. And the CNN-LSTM outperforms the LSTM in predicting vehicle acceleration during car-following scenarios. The findings from this study can enhance the development of personalized functionalities within ADAS to promote its deployment, thereby improving its acceptance and safety.
... This modeling method is marked by its simplicity, ensuring adequate accuracy while significantly enhancing simulation efficiency. Therefore, car-following models have been widely adopted to investigate microscopic traffic flow [33][34][35][36][37]. ...
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With advanced technologies such as perception and automatic control, automated vehicles (AVs) are gradually replacing human-driven vehicles (HDVs). They will become a significant component of transportation systems. As the crucial longitudinal control model for AVs, the existing adaptive cruise control (ACC) model faces challenges such as poor linear stability and compromised safety. To solve the problem, this paper proposes an adaptive cruise control model to avoid collision risk. First, the delay-considerate collision risk control strategy (DCRCS) is developed to consider vehicles' perception and control delay. Then, a proposed adaptive cruise control (PACC) model is designed by incorporating DCRCS into the ACC model based on the control gain coefficient. Finally, the effectiveness of the PACC model is discussed and analyzed in terms of stability, safety, efficiency, energy consumption, and pollutant emissions through a combined approach of theoretical analysis and simulation experiments. The result shows that (1) compared to the classical ACC model, the PACC model demonstrates significant improvements in system stability, reduced safety risks, enhanced efficiency, and decreased energy consumption and pollutant emissions. These enhancements are observed when the control gain coefficient is set to a positive value. (2) The equilibrium speed minimally influences the effectiveness of the PACC model. Under varying speed conditions, the PACC model consistently maintains stable performance. (3) The effectiveness of the PACC model is closely related to the selection of the control gain coefficient. Considering the effects of the control gain coefficient on stability, safety, efficiency, energy consumption, and pollutant emissions of the PACC model, the optimal range for the control gain coefficient is in the range of [0.8, 1.0]. In summary, compared to the classical ACC model, the PACC model exhibits outstanding performance in various aspects. It provides theoretical support for the longitudinal control of AVs in the future.
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The traffic fundamental diagram (FD) describes the relationships among fundamental traffic variables of flow, density, and speed. FD represents fundamental properties of traffic streams, giving insights into traffic performance. This paper presents a theoretical investigation of dynamic FD properties, derived directly from vehicle car-following (control) models to model traffic hysteresis. Analytical derivation of dynamic FD is enabled by (i) frequency-domain representation of vehicle kinematics (acceleration, speed, and position) to derive vehicle trajectories based on transfer function and (ii) continuum approximation of density and flow, measured along the derived trajectories using Edie's generalized definitions. The formulation is generic: the derivation of dynamic FD is possible with any analytical car-following (control) laws for human-driven vehicles or automated vehicles (AVs). Numerical experiments shed light on the effects of the density-flow measurement region and car-following parameters on the dynamic FD properties for an AV platoon.
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The emergence of autonomous vehicles (AVs) could transform traffic flow characteristics and affect energy consumption subsequently. The primary objective of this study is to propose an improved eco-driving strategy for the mixed platoons by exploring the impacts of AVs on energy consumption characteristics. Initially, leveraging precise AV control, this study utilizes a Model Predictive Control (MPC) system to optimize AV trajectories, aiming to minimize energy consumption within mixed platoons of AVs and human-driven vehicles (HDVs). Given the differing collaborative capabilities of HDVs and AVs in mixed platoons, exceptional situations may arise at intersections where not all platoon vehicles cross the stop line during the same green light cycle (GLC). To address these challenges, we then propose a splitting approach to optimize the platoon operation mode. This approach allows for flexible splitting decisions based on AVs market penetration and current traffic conditions. The simulation results indicate that the energy consumption reduction ranges from 15.35% to 35.39% and 2.75% to 3.64% compared to the traditional mode and the eco-driving mode, respectively. In addition, through an analysis of the energy consumption of various vehicle orders within a mixed platoon, we determined that positioning the second AV in the middle platoon maximizes stability, resulting in a more consistent energy consumption pattern. These findings highlight the advantages of the strategy in terms of energy consumption, providing theoretical support management of mixed platoons.
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Connected and Automated Vehicles (CAV) are currently not intelligent enough to deal with complex traffic conditions. CAVs traveling in platoons can reduce external interference and further improve road capacity. Therefore, a mixed traffic capacity gain formula was constructed, considering both communication interaction and size limitations of CAV platoons. CAVs are divided into three types in a mixed traffic environment in terms of different states and positions. A model was constructed to examine how several parameters affect capacity, and these capacity-improving parameters were used to indicate coefficients of headway gains. Additionally, an empirical formula for pollutant emissions with different headway gains is applied in the mixed traffic. A series of microsimulations are tested based on the improved Full Velocity Difference (FVD) model and CAV models which are established as discrete steps based on the above analyses. The simulation results are highly consistent with the theoretical analyses, demonstrating the universality of this model. Finally, it indicated that the overall optimal size of CAV platoons is between 5 and 10 when taking both the road capacity and pollutant emissions into consideration.
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With the development of automated driving technology and communication technology, the emergence of connected automated vehicles (CAVs) will significantly improve traffic efficiency and safety. To study the influence of connected automated vehicles (CAVs) on mixed traffic flow, this study proposes a cellular automata (CA) model of mixed traffic flow considering the driving behavior of the platoon of CAVs. Firstly, three car-following modes, Human-driven vehicle (HDV), Adaptive Cruise Control (ACC), and Cooperative Adaptive Cruise Control (CACC), in mixed traffic flow are analyzed. Secondly, on the bias of the safety distance model and platoon behavior analysis, the acceleration, deceleration, and randomization rules of the cellular automata model of mixed traffic flow are developed. The time step is introduced into the model in the rule design, and the reaction time of the vehicle is considered in the randomization rule. Finally, a numerical simulation is conducted to analyze the fundamental diagram of mixed traffic flow, traffic congestion, and speed volatility under different penetration rates of CAVs. The result shows that (1) the road capacity under the pure CAV environment has increased by 3.24 times compared with the pure HDVs; (2) the maximum congestion reduction percentage can reach 63.36%; and (3) when the penetration rate reaches 80%, the velocity fluctuation decreases significantly. Sensitivity analysis shows that when the penetration rate of CAVs is 100%, the maximum platoon size increase can improve the road capacity. Furthermore, the maximum platoon size has an optimal value in a specific density range.
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Abstract Advances in connected and autonomous vehicles have the promise to reshape the future of the transportation system. How and when the benefits associated with automation and connectivity technology will start to impact the performance of an urban corridor is an issue of interest for traffic operators. This paper proposes an analytical capacity model for urban corridors with mixed traffic based on the concept of macroscopic fundamental diagram. The model incorporates the full spectrum of connected and autonomous vehicle penetration rates as well as the reaction times of different vehicle following patterns. The connected and autonomous vehicle platoon intensity, formulated as an exponential function of the connected and autonomous vehicle penetration rate, is also considered in the proposed analytical capacity model. Numerical experiments are conducted to verify that different reaction time settings yield disparate results. Some reaction time settings were found to cause the corridor capacity to increase monotonically with the connected and autonomous vehicle penetration rate while others led to decreases in corridor capacity with connected and autonomous vehicle penetration rates. Finally, the validity of the proposed methodology is verified via simulation tests in VISSIM 2020.
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As Connected and Automated Vehicles (CAVs) gradually occupy more market share, the mixed traffic flow composed of MVs and CAVs will exist for a long time. This paper mainly explores the impact of CAV platoon management and control mode degradation on the fundamental diagram, and further studies how they affect fuel consumption and pollutant emissions. First, in terms of platoon management, the mixed traffic flow configurations and distribution characteristics of four types of vehicles were proposed. Second, numerical analyses were conducted, and a fundamental diagram considering CAV platoons and degradation of the Cooperative Adaptive Cruise Control (CACC) system was proposed. To validate this fundamental diagram, the Intelligent Driver Model (IDM) and the Connected and Automated Vehicle (CAV) and CACC models proposed by the PATH lab were applied for simulations. Third, the influence of CAV platoons on the traffic environment was explored via micro simulations as well. Conclusions were drawn from the results of the simulations: The mixed traffic can obtain significant capacity gain due to the existence of CAV platoons, especially with high Market Penetration Rate (MPR). Moreover, capacity break-downs occurred in the diagram due to the adjustment of information flow topology, and the break-downs were more prominent with higher MPR. Additionally, CAV platoons can significantly reduce fuel consumption and emissions by 34.7% and 41.4% respectively. The first degradation raises fuel consumption and emissions by 44.5% and 60.1%, while the second one by 12.0% and 16.1%, with the initial vehicle speed set at 33m/s and MPR set at 1.0. Finally, according to the sensitivity analysis, an optimal platoon size of 5 vehicles was suggested for CAV platoon management.
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The application of connected and automated vehicles can significantly reduce traffic congestion, fuel consumption, and transportation emissions. Most existing studies on connected and automated vehicles focus on improving traffic efficiency; the impact on fuel consumption and transportation emissions is not concerned. This study evaluates the influence of connected and automated vehicles on fuel consumption and emissions of mixed traffic flow on the expressway. Firstly, fuel consumption and transportation emissions models are introduced. Secondly, three car-following models are employed to capture the car-following behaviors in the mixed traffic flow. Then, a numerical simulation is designed to investigate the influence of connected and automated vehicles on fuel consumption and transportation emissions of mixed traffic flow. Finally, some factors that impacted fuel consumption and transportation emissions of mixed traffic flow are discussed. The simulation results show that connected automated vehicles can significantly reduce fuel consumption and transportation emissions. The maximum reduction percentages of HC, NOx, CO, and fuel consumption are 24.33%, 27.06%, 37.53%, and 40.58%, respectively, at 100% penetration rate of connected automated vehicles. Moreover, the parameters of the car-following models have a significant influence on fuel consumption and transportation emissions. The result indicates that the design of more minor expected headway would significantly improve the energy-saving and emissions reduction effect of connected automated vehicles.
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Many traffic accidents occur in the merging and diverging area of expressway and ramp, which forms the combination bottleneck of traffic accidents and ramp at the same time, and has a negative impact on traffic flow. Based on the classic KKW (Kerner-Klenov-Wolf) model of three-phase traffic flow theory, this paper introduces traffic accident and defines the priority rule of vehicle lane changing in front of the accident site, proposes the traffic flow model of expressway ramp system under the accident conditions, and simulates and analyzes the impact of traffic accident on the traffic flow under the open boundary condition. It is found that the congestion caused by the accident will not propagates to the upstream basically under the free flow, but will obviously propagates to the upstream under the synchronized flow. Besides, the combined bottleneck effect of accidents and on-ramp can be clearly found under synchronized flow. This work is helpful to understand the mechanism of traffic congestion and provide reference suggestions for traffic control under accident conditions.
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This paper shows that the heterogeneity of drivers’ and vehicles characteristics makes platoons, on average, more string-unstable. However, the string instability degree of unstable platoons is much higher in a homogeneous flow than in a heterogeneous one. These results are based on an L∞ characterization of string stability, which is shown to be the most appropriate one from a traffic safety viewpoint. Mechanisms and conditions are discussed in which an L2 characterization is not able to capture the amplification of a speed drop through a string of vehicles. An analytical sufficient condition for the L∞ string stability of heterogeneous vehicles, which move according to a general class of car-following models, is derived. Above all, a thorough comparison of L∞ and L2 string stability characterizations between a homogeneous and a heterogenous flow, is performed. To this aim, the Lp norms of heterogeneous platoons are calculated within a quasi-Monte Carlo framework. The variability of the Lp norm values due to the platoon length, the equilibrium speed, and the probability distribution model of the uncertain vehicle parameters, is analysed. Overall, it is shown that the platoon stability behaviour sensibly changes with the shape and the correlation structure of vehicle model parameter distributions. Therefore, traffic heterogeneity needs to be modelled in order to correctly characterize the string stability of a mixed traffic flow.
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Urged by a close future perspective of a traffic flow made of a mix of human-driven vehicles and connected, automated vehicles (CAVs), research has recently focused at making the most of CAVs capabilities to mitigate the instability of the whole, i.e. mixed, traffic flow. In all works, however, either the two sub-flows are studied under a simplifying but unrealistic assumption of flow homogeneity, or drivers’ and vehicles heterogeneity is not correctly taken into account within each sub-flow. We show here that the only condition developed so far to study a car-following model string stability for a heterogeneous flow, is inaccurate. Therefore, we propose a methodology to model string stability that considers drivers’ and vehicles heterogeneity, which is the essence of a real traffic. Uncertain transfer functions are introduced to map the probability distributions of car-following model parameters into a L2 stability measure of a mixed and heterogeneous traffic. Specifically, they allow us to move from the stability analysis of a car-following model, or of a controller, to the stability analysis of a traffic flow, as interpreted by that model, or controller. Eventually, several other theoretical contributions on stability analysis are given in the paper, aiming at reconciling approaches from different fields. Among these, a mathematical justification of the equivalence between the asymptotic stability of a closed-loop platoon system – which has been studied through the famous “traffic wave ansatz” on a ring-road – and the L2 stability of an open-loop platoon system.
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The maximum platoon size is a critical parameter in connected and automated vehicle (CAV) platoon configuration. However, the effect of platoon size on the transportation system has not been well-studied. This paper unveils the effect of maximum CAV platoon size in terms of road capacity and traffic flow stability. Specifically, the analytical formulations of the capacity and flow stability are developed considering the maximum platoon size. Simulations are conducted to verify the developed theoretical models. For capacity analysis, both the analytical and simulation results indicate that a larger maximum platoon size can help increase the capacity. However, the increment becomes smaller with the increase of maximum platoon size. For flow stability analysis, the theoretical analysis and microscopic simulation show that smaller maximum platoon size leads to greater traffic flow stabilization. In addition, analysis shows that improvements in capacity and traffic stability are more profound when CAV penetration and platooning intensity are high.