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DEU INTERNATIONAL SYMPOSIUM SERIES ON
GRADUATE RESEARCHES-2022 EngineeringScience
November 17-18, 2022
IZMİR-TÜRKİYE
A DISCUSSION ON POSSIBLE EFFECT OF AUTONOMOUS VEHICLES AT SINGLE-
LANE ROUNDABOUTS IN IZMIR, TURKEY
Ruti R. Politi
Civil Engineering Department, Izmir University of Economics
İzmir, Turkey
ruti.politi@ieu.edu.tr
Zeynel Baran Yıldırım
Civil Engineering Department, Dokuz Eylül University
İzmir, Turkey
zeynelbaran.yildirim@deu.edu.tr
Yağmur Özinal Avşar
Civil Engineering Department, Dokuz Eylül University
İzmir, Turkey
yagmur.ozinal@deu.edu.tr
ABSTRACT: Autonomous vehicles will take an important place among the transportation systems of the
future. Studies on traffic safety and efficiency of vehicles with and without human drivers remain popular. In
this study, the capacity of single-lane roundabouts is investigated using HCM approach by adding autonomous
vehicles to the system with different penetration rates at different traffic volumes. In this context, investigations
were made for the entrance capacity and minimum delay estimations for five single lane roundabout approaches
in İzmir. As a result of the analysis, it was discovered that autonomous vehicles can significantly increase the
minor flow capacity and produce significantly more effective results in reducing the minimum delay values
under conditions of high main flow capacity.
Keywords: roundabout, autonomous vehicles, capacity adjustment factors, critical gap, follow-up time
1. INTRODUCTION
Roundabouts are the intersections which circulates the traffic flow around a center island
(Çalışkanelli, et. al., 2009). Generally, the capacity analysis of roundabouts can be investigated by
using mathematical models and microscopic simulations. The vehicle number which enter to a
roundabout gives the entry capacity of a roundabout. The main concept underlying on capacity is the
gap acceptance model (Suh et. al., 2018).
In the changing world and with the developing technology, the traditional automobile concept is
showing a transformation. This change is inevitable, especially by the legislation to reduce carbon
emissions and various issues such as climate change, traffic congestion and other factors like parking
restrictions (Mu and Yamamoto, 2019). In addition, in a few years, the mixed traffic conditions that
will occur when the autonomous vehicle will share the roads with drivers all around the world, the
development of smart transportation systems attract a lot of attention. By taking advantage of such
systems, it is possible to develop common models that optimize vehicle speeds along an arterial
according to the communication status of autonomous vehicles with each other (V2V: vehicle to
vehicle) and with infrastructure (V2I: vehicle to infrastructure). Thanks to these models,
interconnected self-driving vehicles can pass through intersections in groups without stopping or with
a minimum of delay (Wang et. al., 2020). The impact of autonomous vehicles shows a great potential
in the transportation system. The main topic is to investigate the effect on capacity (Boualam et. al.,
2022). According to the conducted researches, it can be said that autonomous vehicles have a positive
DEUISGR22 Engineering Science, November 17-18, 2022 | Izmir-TÜRKİYE
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impact on transportation system which improve the safety and can reduce the emissions (Boualam et
al., 2022). An autonomous vehicle or a driverless vehicle can be defined as a vehicle that transports
from one point to another without human driver by remote control. It is thought that autonomous
vehicles have faster reaction times than human drivers in most dangerous situations in traffic, thanks
to their communication with the environment and each other. Thus, both the road traffic safety and the
capacity of the roads increase. In the short and medium term, it is expected that there will be positive
effects on traffic capacity if autonomous vehicles are included in the system. However, studies show
that self-driving vehicles can adversely affect traffic flow characteristics in mixed traffic conditions
(traffic conditions where human drivers and autonomous vehicles are together) (Berktaş and Tanyel,
2020; Le Vine et. al., 2016; Li et. al., 2016).
The conflict movement of a roundabout can arrange by the help of the priority rules. By changing the
minimum gap acceptance and related parameters, the calibration of the simulation models is created
and the adaption of the a real or theoretical roundabout which meets the requirement of the operation
characteristics can represent the current capacity models (Suh et al., 2018). To predict the impact of
autonomous vehicles, Highway Capacity Manual (HCM) models for a single roundabout are taken as
an empirical model which is generated by the observations of pure human-driven traffic (Jiang et al.,
2022).
Cao and Zöldy (2019) presented the impact of connected and autonomous vehicles behavior for fuel
consumption. The driving conditions of autonomous vehicles in a roundabout is investigated to
understand the impact of driving behavior on fuel consumption and emissions. Rafael et al., (2022)
thought that autonomous vehicles would bring a new genre for city planning as a technology that will
reshape the concept of mobility, which has important effects for the economy, environment and
society. In this context, the importance of research conducted to predict the effects of autonomous
vehicles and to maximize their benefits. For this reason, in their study, they investigated the potential
difficulties and benefits that may be encountered with the gradual replacement of autonomous
vehicles by different penetration rates (10%, 30%, 50%, 70%, 90% and 100%) on urban roads with
different characteristics. Traffic volumes and their associated effects on air quality are also discussed.
For this purpose, two urban areas with different characteristics such as Porto and Aveiro were selected
for the field study and a traffic model, emission model and local air quality model was developed. The
results revealed that the benefits of autonomous vehicles are directly related with urban design and
road features. In the study conducted in Aveiro, it was found that autonomous vehicles have a positive
change in daily NOx emissions with a decrease of 2.1% compared to the model with 10% driver and
7.7% reduction in the model consisting of fully autonomous vehicles. Parallel to the emission effects,
positive effects on air quality were observed with an average of 4% reduction in NO2 concentrations
in the scenario consisting of fully autonomous vehicles. Boualam et al., (2022) presented the impact
of autonomous vehicles on the capacity with the help of a microsimulation of single‐lane
roundabouts. By conducting different scenarios in VISSIM with changing the input traffic volumes
and penetration rates of autonomous vehicles to determine the queue lengths. The Highway Capacity
Manual (HCM) roundabout model was preferred for making the capacity calculation of the
roundabout which is located in Györ (Hungary). The study concluded that the follow‐up times and
critical gaps showed a decrease as the penetration rate of autonomous vehicles increased. A
penetration rate of 20% and 40% of autonomous vehicles in traffic flow shows a rise in approach
capacities approximately 10% and 20%.
DEUISGR22 Engineering Science, November 17-18, 2022 | Izmir-TÜRKİYE
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2. LITERATURE REVIEW
Generally, the capacity of roundabouts is based on two main methods (Tanyel, 2005):
1. Geometric / Empirical method.
2. Behavioral / Analytical method.
The geometric method examines the interaction between vehicles entering the roundabout at peak
hours. The main purpose of this method is to correlate the regression parameter values with the
geometric parameters of the roundabout. The essential geometric parameters can be listed as the
number of lanes, diameter of central island, entry width, exit width, width of circulating roadway,
number of legs connected to the roundabout, the distance between the entrance and exit points. On the
other hand, the underlying theory of the behavioral method is the driver behavior and critical gap
acceptance methods. The mostly used behavioral method is the critical gap acceptance method. A
driver approaching to the roundabout from the minor flow will encounter a lag as the front entrance
gap if the roundabout is empty. However, if there is a vehicle approaching to the roundabout from the
main flow, and the roundabout is fully occupied, it is needed a gap between these main road vehicles
for entering. This means that the driver coming from the minor flow will only be able to join to the
main flow when a safe interval equal to or greater than the critical gap value in terms of time, such as
"T". The magnitude of the critical gap can also be expressed as a value in terms of the safest
minimum time chosen for the least possible delay. For this reason, it can be said that the performance
of the traffic can be influenced by critical gap and follow-up headway. These values are important
especially at the approach and circulating section of roundabouts with a mixed traffic composition and
volume (Hamim et. al., 2021; Tanyel, 2005).
The studies carried out showed that the equation suggested by Troutbeck (1991) and Hagring (1998)
gave good results for the conditions of our country, so it was also used within the scope of this study
(Tanyel, 2005).
=( )
1
(1)
where
qe = minor flow capacity (veh/s)
qc = major flow capacity (veh/s)
α = proportion of free cars in the mainstream
T = critical gap acceptance value (s)
T0 = follow – up headway of minor flow vehicles (s)
∆ = Interval value in terms of minimum time (s) between vehicles in the main flow
λ = reduction coefficient
It can be seen from the literature that there are different assumptions for α by different researchers.
Especially, for a single-lane roundabout, Tanyel et al. (2007)’s model is used as α value. α is derived
from Cowan M3 distribution by the method of moments. By assuming Δ as 2 sec, α is generated from
regression analysis (Çalışkanelli et al., 2009).
= 1.11 1.47 if 0.07 (2)
= 1.0
DEUISGR22 Engineering Science, November 17-18, 2022 | Izmir-TÜRKİYE
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3. CASE STUDY
This study examines the effect of autonomous vehicle on the capacity of single‐lane roundabouts by
using HCM approach. Different scenarios with different traffic volumes and different penetration
rates of autonomous vehicles realize within the scope of the study. In order to determine the entry
capacity of a single lane roundabout with the integration of autonomous vehicles, a predefined
roundabout parameters were used to make the appropriate calculations. The calculation of entry
capacity of roundabout shown in Equation 3.
= ()
( )+
()
( ) (3)
One of the important performance indicators is delay. A service delay or minimum delay is a very
critical component of the average delay per/vehicle in this study. A formula developed by Tanyel et.
al. (2013) is used for the calculation of the minimum delay to show the effect of autonomous vehicles
on the performance of single-lane roundabouts (Equation 4).
= 0.100 + (83.86 0.100). (4)
The parameters of the roundabout are listed below and more detailed information based on the
autonomous vehicle penetration rate, critical headway and follow-up time proposed by HCM 2016 for
a single lane roundabout are given in Table 1. Six different scenarios according to the different
autonomous vehicle penetration rate (from 0% to 100%) were tested to investigate the effect of the
mixed traffic condition of manually driven vehicles and autonomous vehicles. In scenarios with
autonomous vehicles, HCM manual standard values were used, since there is no available method for
calculating the critical gap and follow-up time (Boualam et. al., 2022).
Table 1. Critical headway and follow-up time proposed by HCM 2016 for a single lane roundabout (Jiang et al.,
2022) .
Scenarios % of
Vehicles
% of Autonomous
Vehicles as a Penetration
Rate
Critical Headway (s) Follow-up Time (s)
1
0
100
4.9
1.9
2
20
80
4.9
2.0
3
40
60
4.9
2.1
4
60
40
4.9
2.2
5
80
20
4.9
2.4
6
100
0
4.9
2.6
Within the scope of the study, investigations were made at the five single-lane roundabout approaches
in Izmir, Turkey, for entry capacity and minimum delay estimations. The critical gap acceptance and
follow-up time values obtained through the observations are given in Table 2. The critical gap
acceptance value for drivers on the minor flow should be calculated based on the geometry of the
roundabout. However, in this paper, this value is taken as T = 5.13 seconds. Moreover, the follow-up
time of the drivers is taken as 2.84 seconds and the minimum time interval between vehicles on the
highway is also taken as ∆ = 1.8 seconds.
Table 2. The gap acceptance and follow-up time of observed roundabouts (Tanyel et. al., 2013).
Critical gap-T(s)
Follow-up time-To (s)
Roundabouts
Average
Variance
Average
Variance
Bostanlı 1
4,80
5,3
2,9
1,1
Bostanlı 2
5,3
0,7
2,9
0,3
Soğukkuyu
6,2
2,1
2,9
1,4
Ege Rektörlük
4,6
3,1
2,9
1,5
Gündoğdu 1
4,7
4,4
2,9
1
Gündoğdu 2
5,8
4,1
2,8
2
Sanayi
4,5
3,1
2,6
1,1
Total Average
5,13
3,26
2,84
1,2
DEUISGR22 Engineering Science, November 17-18, 2022 | Izmir-TÜRKİYE
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4. RESULTS AND DISCUSSIONS
As a result of the analysis, it was seen from Figure 1 that the existence of autonomous vehicles significantly
increase the minor flow capacity. As expected, the rate of increase of the minor flow capacity under low
circulating flow conditions is greater than for the main flow. The main reason for this situation is that while
autonomous vehicles enter the main road, it is affected by the traffic volume on the main road. Another
limitation is that the analyzes were made by assuming that all the drivers on the highway were human drivers. In
addition, only the critical gap acceptance value and follow-up time values were changed in this study.
Figure 1. Capacity curves for single-lane roundabout with different penetration rates.
Delays are expected to decrease significantly if autonomous vehicles enter the network. The reduction rate of
delay is observed lower at the points where the traffic volume on the highway is low and more at the points
where it is high (Figure 2). This situation allows autonomous vehicles to enter the network at relatively smaller
intervals, even if the number of vehicles in the main flow is high, if they have the appropriate equipment. As a
result, delays are reduced. Although it is in line with the results made with capacity, it can be said that
autonomous vehicles actually give more beneficial results in terms of reducing the delays of vehicles on the
minor flow.
Figure 2. Delay curves for single-lane roundabout with different penetration rates.
DEUISGR22 Engineering Science, November 17-18, 2022 | Izmir-TÜRKİYE
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5. CONCLUSIONS
In this study, the effects of autonomous vehicles on single-lane roundabouts were investigated. For
this purpose, the critical gap and follow-up time range values recommended by HCM 2016 were
taken as a basis. As a result of the analysis, it was seen that autonomous vehicles can significantly
increase the minor flow capacity, as expected. Additionally, under conditions of high major flow
capacity, it has been discovered that it produces results that are significantly more effective in
reducing the minimum delay values. This study is limited to the data of five single-lane roundabouts
in the province of Izmir and the studies conducted in the USA. The cases where autonomous vehicles
are present in the major flow are not examined within the scope of the study. Future work will aim to
increase the number of observed roundabouts, conduct more detailed analyses of autonomous
vehicles, and investigate scenarios involving autonomous vehicles moving in a roundabout.
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