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RESEARCH ARTICLE
The impact of mobile phone use on where we
look and how we walk when negotiating floor
based obstacles
Matthew A. Timmis
1
*, Herre Bijl
1
, Kieran Turner
1
, Itay Basevitch
1
, Matthew J. D. Taylor
2
,
Kjell N. van Paridon
1
1Cambridge Centre for Sport and Exercise Sciences (CCSES), Department of Sport and Exercise Sciences,
Anglia Ruskin University, Cambridge, United Kingdom, 2Centre for Sports & Exercise Sciences, School of
Biological Sciences, University of Essex, Colchester, United Kingdom
*Matthew.Timmis@anglia.ac.uk
Abstract
Pedestrians regularly engage with their mobile phone whilst walking. The current study
investigated how mobile phone use affects where people look (visual search behaviour) and
how they negotiate a floor based hazard placed along the walking path. Whilst wearing a
mobile eye tracker and motion analysis sensors, participants walked up to and negotiated a
surface height change whilst writing a text, reading a text, talking on the phone, or without a
phone. Differences in gait and visual search behaviour were found when using a mobile
phone compared to when not using a phone. Using a phone resulted in looking less fre-
quently and for less time at the surface height change, which led to adaptations in gait by
negotiating it in a manner consistent with adopting an increasingly cautious stepping strat-
egy. When using a mobile phone, writing a text whilst walking resulted in the greatest adap-
tions in gait and visual search behaviour compared to reading a text and talking on a mobile
phone. Findings indicate that mobile phone users were able to adapt their visual search
behaviour and gait to incorporate mobile phone use in a safe manner when negotiating floor
based obstacles.
Introduction
The number of people who own a mobile (cell) phone has increased dramatically in the last 30
years. In 1985, approximately 340,000 people owned a mobile phone in the United States (US).
In 2010, this had risen to 302.9 million [1]. Recent surveys suggest that over 85% of people in
the US and ~77% of the world’s population now own a mobile phone [2–4].
People engage with their mobile phone in a variety of ways, for example, making a tele-
phone call, reading and sending text messages and emails, and engaging in online activities
such as social networking and watching videos [5]. This increase in functionality resulted in
2.1 trillion manualized text messages sent, 2.2 trillion phone minutes used, [1] and 897 million
people sending emails from their phone, something which is expected to rise to ~1.78 billion
in 2017 [6].
PLOS ONE | https://doi.org/10.1371/journal.pone.0179802 June 30, 2017 1 / 20
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OPEN ACCESS
Citation: Timmis MA, Bijl H, Turner K, Basevitch I,
Taylor MJD, van Paridon KN (2017) The impact of
mobile phone use on where we look and how we
walk when negotiating floor based obstacles. PLoS
ONE 12(6): e0179802. https://doi.org/10.1371/
journal.pone.0179802
Editor: David J Clark, University of Florida, UNITED
STATES
Received: February 7, 2017
Accepted: June 5, 2017
Published: June 30, 2017
Copyright: ©2017 Timmis et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the paper and its Supporting Information
file.
Funding: The authors received no specific funding
for this work
Competing interests: The authors have declared
that no competing interests exist
Pedestrians regularly engage with their phone in a variety of ways whilst on the move.
Approximately 62% of Americans report that they use their phone whilst ‘on the go’ [7]. Data
on serious injuries involving road traffic and mobile phone use is sparse. Analysis of the
National Electronic Injury Surveillance System (NEISS) for emergency departments between
2000–2011 identified 5,754 cases of emergency department admissions related to mobile
phone use. Of these admissions, 310 were mobile phone related injuries [8] and 78% of the
mobile phone related injuries were the result of falls [8]. In 2010, over 1,500 pedestrians visited
hospitals in the US due to tripping, falling or walking into something whilst using their mobile
phone. This was a marked increase from the number in 2007 (597 reported visits) and almost
four times that of 2006 [9]; these figures are likely underestimated due to people attending the
hospital failing to report phone use as the cause of the accident or not requiring hospital treat-
ment for minor injuries [9].
The increase in pedestrian-phone related accidents has subsequently led to greater law
enforcement for pedestrians crossing roads illegally when texting in several US towns and
states [10] and in China, segregating footpaths for people walking and using their mobile
phone and those walking without using a mobile phone [11].
The increase in pedestrian related accidents when walking and engaging with a mobile
phone has led researchers to investigate the effect of phone use on pedestrian safety. Compared
to walking without a mobile phone, adaptations in walking gait have been reported when text-
ing, reading and talking on a mobile phone [3,12–16]. Phone use results in pedestrians walking
slower, deviating more from a straight line or changing direction more, and demonstrating
reduced situation awareness and/or inattentional blindness [3,12–16]. Importantly, the afore-
mentioned gait research all required participants to walk along level terrain. In everyday life
we frequently encounter complex terrain requiring adaptions in gait, for example negotiation
of a change in surface height such as a step or kerb. There is currently little research investigat-
ing the impact of mobile phone use on adaptive gait.
The changes in walking gait that occur as a result of mobile phone use may be attributed to
the altered visual search behaviour when engaging with the phone. Whilst walking and looking
at the phone’s screen, pedestrians will not be able to acquire concurrent visual information
from the fovea (central part of the eye which provides the highest level of visual acuity) of
the surrounding environment to guide locomotion: something which has been previously
highlighted as important in safe walking (e.g. [17,18]). When walking and texting or reading a
text, the fovea is fixated on the phone’s screen. To acquire precise visual information from the
environment, the eyes will need to re-fixate from the phone’s screen to an area within the envi-
ronment. If this re-fixation does not occur frequently enough or long enough to sufficiently
acquire an updated visual representation of the environment, an increased risk of accidents
are expected as potential hazards will not be seen or seen without allowing enough time to
plan/initiate a suitable response (i.e. walk round the hazard or step over the obstacle). Indeed,
visual information is required in the previous step to successfully implement an adaptive strat-
egy to safely step over or under an obstacle, and is required at least 2 steps in advance for
changing direction (c.f. [19]). However, previous research has demonstrated that safe travel is
still possible when a large proportion of time (~40%) is spent fixating at task irrelevant objects
[20] or through relying on peripheral vision (aspect of vision which does not encompass the
fovea) to step over an obstacle [21] or step up onto a surface height change [22] and when
using a mobile phone, navigate whilst cycling [23] driving [24,25] and walking [14,16].
The current research addresses two important gaps in the literature;
1. Investigates whether adaptive gait is affected when required to walk up to and step onto a
raised surface when engaging with a mobile phone (talking, texting or reading a text)
The impact of mobile phone use on where we look and how we walk when negotiating floor based obstacles
PLOS ONE | https://doi.org/10.1371/journal.pone.0179802 June 30, 2017 2 / 20
compared to no phone being present. Much of the current literature has investigated how
people navigate their environment through avoiding obstacles (i.e. route planning) whilst
using their phone (e.g. [3,12–14,16,26,27]. Currently, there are few research studies investi-
gating the effect of mobile phone use on obstacle negotiation. Negotiating an obstacle is a
task that presents a high risk of injury since positioning of the foot in relation to the obstacle
[28], the speed (velocity) and clearance height of the swinging limb when crossing the rising
edge of the object [18] and foot placement on the upper level [29] all impact the risk of trip-
ping / falling.
2. Investigates whether visual search behaviour changes dependent upon the manner in which
the phone is being used. It is relevant to note that Oulasvirta et al. [30] highlighted that
when using a mobile phone to search the internet, pedestrians looked at their screen in
approximately four second bursts and made use of the time when the web browser was
loading to acquire visual information from the surrounding environment. It is likely that
when reading a text or email, rather than employ one long fixation, people will employ a
series of shorter fixations to ‘glance’ at their phone to read whilst frequently fixating at the
surrounding environment. However, with no previous research in this area, it is currently
not known how adaptations in visual search strategy affect the pedestrian’s ability to safely
negotiate floor based obstacles compared to no phone being present.
With the visual information acquired in advance of walking up to and negotiating a surface
height change potentially impacting adaptive gait (i.e. negotiation over the floor based obsta-
cle), we hypothesised that pedestrians will alter their visual search behaviour, which will subse-
quently impact their adaptive gait when using a phone compared to no phone being present.
Furthermore, tasks which require the individual to fixate predominantly at the phone’s screen
(i.e. typing a text and reading a text) will have a greater effect on visual search behaviour and
adaptive gait.
Method
Participants
Twenty one participants (16 male, 5 female, age 25.4±6.2 years, height 178±14 cm, mass 78.0±
15.4 kg, BMI 23.9±2.8 kg/m
2
; mean±SD) were recruited to the study. Participants were re-
cruited via opportune sampling according to the following inclusion criteria. Participants, by
self-report, were fit and healthy with no history of neurological or musculoskeletal disorders
which could affect balance or gait and had either normal or corrected to normal vision (through
wearing contact lenses) as determined through self-report: Participants were excluded if they
wore spectacles as this interfered with eye tracking quality. All participants habitually used a
touch screen smart phone, had used their current phone for a minimum of 6 months (range
0.5–3+ years) and all reported frequently using their mobile phone for texting, talking and read-
ing a text whilst walking. Participants used their own phone in the study. The tenants of the
Declaration of Helsinki were observed and Anglia Ruskin University’s Ethical Committee
approved the study. Written consent was obtained from each participant prior to participation.
Protocol
Experimental setup. Participants walked along a 5.6m walkway, negotiating an obstacle
and a surface height change. The obstacle, constructed from medium density fibreboard (0.5m
width [medial-lateral dimension], 0.13m high [vertical dimension] and 0.012m deep [anterior-
posterior dimension]), was positioned 1.5m from the start position. A step-up box (York Bar-
bell, USA, 0.61m wide, 0.075m high and 0.61m deep) was used as the surface height change.
The impact of mobile phone use on where we look and how we walk when negotiating floor based obstacles
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The step-up box was positioned 3.95–4.05m from the start position and was constructed from
plywood with a 1.27cm black rubber non-slip top. A black non-slip mat (width 1.5m length
15m) was laid on a light grey laboratory floor to indicate the walking path. Both the obstacle
and surface height change were placed in the centre of the path (Fig 1).
The variation of +/- 10 cm in surface height change location was to ensure that participants
did not become habituated with identifying and negotiating the surface height change located
at the same distance from their start position.
A number of walking only trials (presented every third trial; a 1:3 ratio) were also com-
pleted. During these walking only trials, both the obstacle and surface height change were
removed from the walkway, creating a level walkway. These trials were included to reduce the
likelihood of participants becoming habituated.
Procedure. Prior to data collection, participants were instructed that for each trial, they
were required to walk along the walking path where there may or may not be an obstacle pres-
ent. Participants were told to safely step over the obstacle and avoid contacting / knocking over
the hazard. Prior to the start of each trial, participants faced the opposite way from the intended
walking direction. This particular start position was used to ensure participants did not receive
advanced visual information from the environment prior to the start of the trial. Participants
were told which of the following phone conditions they would complete immediately prior to
the onset of the trial. Of note, for each phone condition, participants were not given any specific
instruction regarding how to hold / interact with the phone. This was a purposeful strategy to
ensure participants interacted with their phone in a manner reflective of being outside of the
laboratory. The only specific instruction related to when talking on the phone (talk condition).
Participants were instructed to keep the phone next to their ear and talk instead of using the
speaker phone function. At the onset of the trial, the participant was free to select which foot to
start (initiate gait) with (which varied from trial to trial). All participants completed the four dif-
ferent phone conditions three times, in a fully randomised order (i.e., 12 total trials);
1. No phone- Walking without a phone (phone was placed in the participant’s pocket).
2. Talking on their phone- at the start of the trial, participants were asked a question which
they were required to answer whilst completing the gait task. Simple questions were asked
which ranged in length from 23 to 42 (34±7, mean ±SD) characters, including spaces (e.g.
‘Have you seen any good films lately?’ or ‘What is your favourite type of music?’). During
this condition, participants spoke into the phone to answer these questions.
3. Read a text message–prior to the start of the trial, the experimenter sent a text message to
the participant’s phone. Participants began the trial immediately upon opening the text
Fig 1. Schematic of the experimental set-up.
https://doi.org/10.1371/journal.pone.0179802.g001
The impact of mobile phone use on where we look and how we walk when negotiating floor based obstacles
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message. Participants were required to read the text message whilst completing the gait task
and upon completion of the trial, relay the message back to the experimenter. Sentences
ranged in length from 23 to 38 (31±6 mean ±SD) characters, including spaces (e.g. ‘Shall
we go for pizza?’ or ‘Are you watching the football tonight?’). At the end of the trial, partici-
pants were required to relay the message back to the experimenter to ensure they had cor-
rectly engaged with the phone task.
4. Write and send a text message–Prior to the start of the trial, the experimenter told the par-
ticipant what message to write on their phone. The participant verbally repeated what they
were required to write, confirming they had heard the message correctly. Participants were
required to write the text message whilst completing the gait task and send the text (to the
experimenter’s phone) prior to completing the trial. At the end of the trial, the experi-
menter read their phone to confirm that the participant has sent the message correctly. The
texting keyboard was the typical QWERTY keyboard. Predictive text was permitted. Sen-
tences ranged in length from 15 to 36 (27±8 mean ±SD) characters, including spaces (e.g.
‘Shall we meet at the train station?’ or ‘What shall we do today?’).
The content for the talk,read and write phone conditions was similar in length and com-
plexity to previous research e.g. [13,14]. Within the talk,read and write condition, participants
were instructed to only start using their phone after they had turned around and started
walking.
Of note, trials were the participant had completed the task (talk,read or write) on their
phone prior to negotiating the surface height change were disregarded and repeated using a
longer sentence. This only occurred twice for one participant.
Equipment
Visual search behaviour was recorded using an SMI iViewETG head mounted mobile eye
tracker (SensoMotoric Instruments Inc, Warthestr; Germany, Ver. 1.0) and was sampled at 30
Hz. The eye tracker contains 3 cameras built into the glasses, an infrared camera to record
movements of each eye and a high definition camera (24 Hz) to record the visual scene. The
eye tracker also contains an integrated microphone to record audio. Data from the eye tracker
were recorded on a mini laptop (Lenovo X220, ThinkPad, USA) with iView ETG (Ver. 2.0)
recording software installed. A three point eye calibration was performed to verify point-of-
gaze and the calibration was checked following every third trial. The spatial resolution of the
system was 0.1˚, with gaze position accuracy of ±0.5˚. The laptop was placed in a backpack
which was worn by the participant during testing. None of the participants reported that wear-
ing the backpack affected their balance whilst walking.
Three-dimensional kinematic data were sampled at 100 Hz using a motion capture system
(Codamotion movement analysis system; Charnwood Dynamics Ltd, UK). Three coda units
were positioned around the laboratory to create a 360˚ capture volume as the participant nego-
tiated the surface height change. Active markers were attached bilaterally, to the superior
aspects of the second metatarsal head, the most distal, superior aspect of the second toe, the lat-
eral malleoli, the posterior aspect of the calcanei, the sternum and antero-lateral and postero-
lateral aspects of the head.
Electronic timing gates (Smart-Speed, Fusion Sport, Australia) were positioned at the start
and end of the 5.6m walkway. As the participant walked past the timing gates, a single ‘beep’
was emitted. The auditory tone recorded by the eye tracker provided the trial length and start
and end points to begin tracking the visual search data.
The impact of mobile phone use on where we look and how we walk when negotiating floor based obstacles
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Data analysis
Visual search. Point of gaze data from the eye tracker was analysed offline using BeGaze
software (Ver.3.4) and was subject to frame by frame analysis. Each trial was tracked from the
first frame the auditory noise from the light gates (denoting the start of the trial) was registered
on the eye tracker’s microphone up until the second auditory noise from the light gates was
registered (denoting the trial being complete).
Areas of interest (AOI) were used to define key locations within the visual scene and com-
prised of; Phone, Intended travel path, Surface height change and ‘‘Other” (denoting fixations
to task-irrelevant locations within the display). Intended travel path was defined as the area
down in front of the participant on the walkway and when looking straight ahead to provide
information regarding direction of travel. Each point of gaze in the real-time dynamic visual
scene was mapped manually (frame by frame) to the AOIs. Fixations were determined as four
or more consecutive frames (120 ms) to an area of interest; a threshold consistent with previ-
ous research used to define a fixation (e.g. [31]).
The following variables were used to analyse eye tracking data;
1. Trial length–see description above.
2. Relative number of fixations on each AOI–Higher number of fixations to a particular AOI
provides an indication of the AOI’s relative relevance to information processing and subse-
quent task execution [32]. Calculated as a percentage overall trial length to account for any
differences in actual trial length between conditions.
3. Relative fixation time on each AOI–Longer time spent fixating at a particular AOI allows
more information to be obtained, indicating greater relevance to information processing
and subsequent task execution [32]. Calculated as a percentage overall trial length to
account for any differences in actual trial length between conditions.
Gait. Negotiation of the initial obstacle was not analysed. The obstacle was placed in the
walkway in an attempt to make the task increasingly realistic since in everyday life we fre-
quently negotiate multiple obstacles in our travel path. Analysis focussed on how both left and
right limbs were positioned on the floor in the final step immediately prior to negotiating the
surface height change, negotiation of the surface height change and the placement of each foot
after crossing the surface height change (Fig 2). Due to the foot (left or right) participants initi-
ated gait with at the start of the trial altering throughout the study, the lead foot which stepped
Fig 2. Representation of foot placement and clearance parameters for the lead and trail foot during
negotiation of the surface height change.
https://doi.org/10.1371/journal.pone.0179802.g002
The impact of mobile phone use on where we look and how we walk when negotiating floor based obstacles
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up onto the surface height change was not consistent. Therefore, the following variables were
not assigned to each (left or right) limb, rather ‘lead’ and ‘trail’ limb to reflect the first and sec-
ond foot stepping up onto the surface height change. The following variables were analysed as
they are considered important to assess the kinematics of gait and changes in such variables
have been shown to increase the risk of tripping / falling [33];
1. Lead and trail limb foot position prior to crossing the rising edge of the surface height
change. Calculated as the anterior-posterior distance between the foot and the front upper
edge of the surface height change.
2. Lead and trail foot stride length–anterior-posterior distance between the lead foot when
planted on the floor (final step prior to negotiating surface height change) and when
planted on the surface height change (lead foot stride length). Anterior-posterior distance
between the trail foot when planted on the floor (final step prior to negotiating surface
height change) and when planted on the floor after negotiating the surface height change
(trail foot stride length, Fig 2).
3. Lead and trail limb vertical toe clearance–Vertical distance between the upper edge of the
surface height change and toe as it crosses the rising edge of the surface height change
(Fig 2).
4. Lead and trail limb horizontal toe velocity—Calculated at the point of vertical toe clearance
(lead and trail limb respectively).
5. Lead and trail limb foot position after crossing the rising edge of the surface height change.
Calculated as the anterior-posterior distance between the foot and the front upper edge of
the surface height change.
6. Head Flexion. Head flexion was calculated at penultimate foot contact with the floor, final
foot contact with the floor (both prior to negotiating the surface height change), instance of
lead vertical toe clearance, and at lead and trail foot contact with the floor after crossing the
surface height change. Head flexion was normalized such that 0˚ was looking straight
ahead. Positive head flexion indicated looking down towards the ground, and negative head
flexion indicated looking up towards the ceiling.
7. Medial-lateral bivariate variable error (BVE). Medial-lateral bivariate variable error (BVE)
was calculated to determine how much each participant deviated from a straight travel path
during each condition. For each trial, BVE was calculated as the difference between the
absolute medial-lateral distance of the sternum marker for each frame (xi) to the mean posi-
tion during the trial (xm);
Medial lateral bivariate error ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
n
X
n
i¼1ðxixmÞ2
s
Statistical design
All participants completed each phone condition (no phone,talk,read, and write), three times,
in a fully randomised order. For the analysis of visual search data, from the 21 participants col-
lected, 4 participants were excluded from the analysis due to a tracking ratio below 90% [34].
The tracking ratio considers the number of video frames whereby no data was recorded by the
eye tracker (i.e. ability to track eye movement was lost). A higher tracking ratio means that
more frames were recorded (conversely, fewer frames relating to eye movements were lost).
The impact of mobile phone use on where we look and how we walk when negotiating floor based obstacles
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An acceptable tracking ratio of 90% was used to ensure that only reliable eye-tracking data
were included. All 21 participants were included in the gait analysis.
For the analysis of visual search data, only the first trial among the three at a given phone
condition was used whereby a participant recorded a tracking ratio above 90% and good pre-
post calibration [34]. This resulted in a total of 68 observations (17 participants x 4 phone con-
ditions x 1 occurrence) being included for statistical analysis.
A random selection of trials from five participants (29%) was coded by two researchers
(KvP and HB) to assess gaze behaviour inter-rater reliability. An acceptable average intraclass
correlation coefficient of r= 0.89 was reached for the frame by frame mapping of time looking
at the phone (r= 0.99), surface height change (r= 0.86) and intended travel path (r= 0.93).
A total of 252 observations were recorded from the analysis of gait data (21 participants x 4
phone conditions x 3 occurrences), however, because the analysis of gait trials was an average
across the three occurrences (trial repeats), 84 data values were utilised for statistical analysis.
To ascertain whether the consistency with which the surface height change was positioned
from the start position affected results, analysis was conducted on the repetition effect and
between the time taken to complete the first, twelfth (mid) and final (last) trial.
Levene’s test for equal variance and the Kolmogorov-Smirnov test confirmed equal vari-
ance and normality of the data (p >0.05). Data were analysed using a one-way repeated mea-
sures ANOVA with phone (no phone, write, read, and talk) as the independent variable. Level
of significance was accepted at p <0.05. Post-hoc analysis where appropriate was completed
using pairwise comparisons (Bonferroni correction). Effect sizes were calculated using Partial
Eta squared η
p2
.
Results
Visual search
There was a significant effect of phone condition on trial time (p <.001). Trial time was signif-
icantly longer in the write compared to all other conditions (Table 1). The increase in trial
time in write compared to read,talk and no phone conditions represented a 67%, 83% and
118% change respectively. Trial time was also significantly longer in read and talk compared to
no phone (31% change and 19% change for read and talk respectively) and read was signifi-
cantly longer than talk (10% change).
Relative number of fixations. Statistical analysis of the relative number of fixations on
the phone in no phone and talk conditions are not reported due to this area of interest not
being present during the trial, subsequently resulting in comparison between these conditions
being redundant. There were significantly more fixations (46% increase) on the phone in the
write compared to read condition (p <.05, Table 1).
There was a significant effect of phone condition on the relative number of fixations on the
surface height change (p <.05) and intended travel path, (p <.001). Post hoc analysis showed
that significantly more fixations were made to the surface height change (Fig 3A) and intended
travel path in the no phone compared to all other conditions (Table 1). The increase in number
of fixations at the surface height change in no phone compared to talk,read and write condi-
tions represented a 40%, 61% and 60% change respectively and at the intended travel path in
no phone compared to talk,read and write conditions represented a 27%, 45% and 51% change
respectively. There were also significantly more fixations (33% increase) on the intended travel
path in the talk compared to the write condition.
There was a significant effect of phone condition on the relative number of fixations on
Other locations (p <.001). There were significantly more fixations on Other locations in
talk compared to no phone,read and write conditions (Table 1). The increase in number of
The impact of mobile phone use on where we look and how we walk when negotiating floor based obstacles
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fixations at other task-irrelevant locations in talk compared to no phone,read and write condi-
tions represented a 40%, 53% and 80% change respectively. There were significantly more fixa-
tions on Other locations in the no phone (68% increase) and read (58% increase) compared to
write condition.
Relative fixation time. Statistical analysis of the relative fixation time on the phone in no
phone and talk conditions are not reported due to this area of interest not being present during
the trial, subsequently resulting in comparison between these conditions being redundant.
There was significantly longer relative fixation time (45% increase) on the phone in the write
compared to read condition (p <.001, Table 1).
There was a significant effect of phone condition on the relative fixation time on the surface
height change (p <.001). There was significantly longer relative fixation time on the surface
height change in the no phone compared to talk,read and write conditions (Fig 3B). The increase
in fixation time at the surface height change in no phone compared to talk,read and write condi-
tions represented a 43%, 79% and 91% change respectively. There was significantly longer relative
fixation time on the surface height change in the talk compared to read and write conditions, rep-
resenting a 64% and 84% increase in talk compared to read and write respectively.
A significant effect of phone condition on the relative fixation time on the intended travel
path was found (p <.001). There was significantly longer fixation time on the travel path in
the no phone and talk compared to both read and write conditions. The increase in fixation
time on the intended travel path in the no phone was 68% and 89% higher compared to read
and write respectively. The increase in fixation time in the talk condition was 61% and 86%
higher compared to read and write respectively. There was significantly longer fixation time
(65% increase) on the intended travel path in the read compared to write condition.
Table 1. Visual search parameters as a function of phone task. Data presented are the group mean (standard deviation).
No phone Talk Read Write P value n
p2
Trial Time (s) 4.79 5.71 6.27 10.46 p<0.001 .714
(.64) (.57) (1.37) (3.23)
No. fixations (%)
Phone - - 37.83 55.34 p <.05 .730*
(23.27) (24.75)
Surface Height Change 17.07 10.24 6.73 6.77 p <.05 .189
(14.52) (7.42) (6.58) (11.06)
Intended Travel Path 51.16 37.35 28.37 25 p<0.001 .375
(19.20) (13.86) (17.33) (18.60)
Other 29.74 49.22 23.05 9.62 p<0.001 .475
(25.50) (21.56) (22.20) (13.81)
Fixation time (%)
Phone - - 60.97 88.18 p<0.001 1.51*
(21.57) (13.47)
Surface Height Change 16.95 9.74 3.55 1.51 p<0.001 .436
(13.71) (9.46) (5.36) (2.22)
Intended Travel Path 57.02 46.49 18.02 6.3 p<0.001 .745
(20.29) (15.24) (14.94) (9.52)
Other 16.25 28.38 8.14 0.01 p<0.001 .467
(20.86) (18.94) (11.02) (.01)
NB. ‘-‘ indicates no fixation.
*denotes effect size calculated using Cohen’s d.
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There was a significant effect of phone condition on the relative fixation time on Other
task-irrelevant locations (p <.001). Post hoc analysis showed that there was significantly
Fig 3. Relative number (a, top figure) and length (b, bottom figure) of fixations at the surface height change in each
phone condition (group mean ±SE). Fig 3 top and bottom—No phone is significantly different to talk, read and write
conditions. Fig 3 bottom—Talk is also significantly different to read and write conditions.
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longer fixation time on Other in the talk compared to no phone,read and write conditions
(Table 1). The increase in fixation time in talk compared to no phone,read and write conditions
represents a 43%, 71% and 100% change respectively. There was significantly longer fixation
time on Other in the no phone and read compared to write condition. The increase in fixation
time in the no phone and read condition was 100% higher (both conditions) compared to write.
Gait
There were no trips or contacts made with the surface height change by any participant
throughout the study. There was no significant effect of repetition on any of the gait variables
(p >.05). Furthermore, there was no significant effect of trial number on time to complete
the trial (p = .418). Average trial time for the first, mid and final trials were 7.51 ±3.65s,
6.53 ±2.33s and 7.13 ±3.01s respectively.
Prior to crossing, there was a significant effect of phone condition on lead limb foot posi-
tion in relation to the front edge of the surface height change (p <.001). The foot was posi-
tioned significantly closer to the front edge of the surface height change in write compared to
no phone, talk and read conditions and represented a 37%, 24% and 21% change respectively.
Prior to crossing, lead limb foot position was significantly closer to the front edge of the surface
height change in the read compared to no phone condition, representing an 18% change.
Prior to crossing, there was a significant effect of phone condition on trail limb foot posi-
tion in relation to the front edge of the surface height change (p <.001). The foot was posi-
tioned significantly closer to the front edge of the surface height change in write compared to
no phone, talk and read conditions and represented an 11%, 11% and 3% change respectively.
Prior to crossing, trail limb foot position was significantly closer to the front edge of the surface
height change in the read compared to no phone and talk condition, representing an 8%
change for both conditions.
There was a significant effect of phone condition on lead stride length (p <.001) and trail
stride length (p <.001). Both lead and trail stride length was significantly shorter in write com-
pared to all other conditions (Table 2). The reduction in lead stride length in write compared
to no phone,talk and read conditions represented a 38%, 28% and 26% change respectively. In
trail stride length, write compared to no phone,talk and read conditions represented a 44%,
35% and 36% change respectively. Lead stride length was also significantly shorter in talk and
read compared to no phone, which represented a 14% and 16% decrease in talk and read
(respectively) when compared to no phone condition. Trail stride length was significantly
shorter in talk (15% reduction) and read (13% reduction) compared to no phone condition.
There was a significant effect of phone condition on lead toe clearance (p <.001), lead toe
crossing velocity (p <.001) and trail toe crossing velocity (p <.001, Table 2). Post hoc analysis
indicated that the lead foot was lifted significantly higher and both lead and trail foot signifi-
cantly slower over the edge of the surface height change in write compared to all other condi-
tions (Table 2,Fig 4). The increase in lead toe clearance in write compared to no phone,talk
and read conditions represented an 18%, 15% and 12% change respectively. The reduction in
lead toe crossing velocity in write compared to no phone,talk and read conditions represents a
40%, 26% and 32% change respectively. The reduction in trail toe crossing velocity in write
compared to no phone,talk and read conditions represented a 38%, 27% and 27% change
respectively. Lead toe crossing velocity was also significantly slower in talk compared to no
phone and read condition, representing a 22% and 8% reduction in talk when compared to
no phone and read conditions respectively. Trail toe crossing velocity was also significantly
slower in the read (16% reduction) and talk (16% reduction) compared to no phone condition
(Table 2).
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There was a significant effect of phone condition on trail toe clearance (p = .025). The trail
foot was lifted significantly higher in read compared to no phone and talk conditions (Table 2).
This represented a 15% and 11% increase in read when compared to no phone and talk condi-
tions respectively.
After crossing, there was a significant effect of phone condition on lead limb foot position
in relation to the front edge of the surface height change (p <.001). The foot was positioned
significantly closer to the front edge of the surface height change in write and talk compared
to no phone and read conditions (write compared to no phone and read, 41% change and
38% change respectively; talk compared to no phone and read, 36% change and 50% change
respectively).
After crossing, there was a significant effect of phone condition on trail limb foot position
in relation to the front edge of the surface height change (p <.001). The foot was positioned
significantly closer to the front edge of the surface height change in write,talk and read
Table 2. Gait parameters as a function of phone task condition. Data presented are the group mean (standard deviation).
No Phone Talk Read Write P value n
p2
Lead limb vertical toe clearance (mm) 120 123 127 142 p<0.001 .295
(22) (23) (30) (28)
Lead limb horizontal toe velocity (m.s
-1
)3.84 3.15 3.4 2.32 p<0.001 .731
(.51) (.53) (.46) (.59)
Trail limb vertical toe clearance (mm) 84 87 97 90 p = .025 .150
(21) (23) (28) (28)
Trail limb horizontal toe velocity (m.s
-1
)3.09 2.61 2.61 1.91 p<0.001 .638
(.67) (.44) (.39) (.41)
M/L BVE (m) 0.57 0.72 0.61 1.35 p<0.001 .361
(.50) (.73) (.44) (.73)
Lead step length (m) 1.29 1.11 1.08 0.8 p<0.001 .690
(.14) (.13) (.17) (.18)
Trail step length (m) 1.26 1.07 1.09 0.7 p<0.001 .751
(.15) (.17) (.17) (.25)
Lead limb foot position (m) 0.22 0.14 0.21 0.13 p<0.001 .399
(.07) (.08) (.07) (.08)
Trail limb foot position (m) 0.94 0.75 0.84 0.49 p<0.001 .694
(.11) (.17) (.16) (.23)
Head angle (˚)
Penultimate foot contact 1 2 22 32 p<0.001 .725
(9) (12) (15) (11)
Final foot contact -1 0 17 34 p<0.001 .670
(11) (15) (19) (13)
Instance of lead toe clearance 2 1 20 32 p<0.001 .680
(11) (14) (15) (11)
Lead foot contact after crossing 0 1 18 32 p<0.001 .188
(11) (14) (17) (11)
Trail foot contact after crossing -10 -7 7 31 p<0.001 .764
-9 -12 -16 -12
NB. Head angle was normalised such that 0˚ indicates looking straight ahead. Negative head angle values indicate looking up and positive values looking
down. Negative foot placement values indicate placement prior to the front rising edge; a larger negative value indicates further away from the front rising
edge
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compared to no phone condition (Table 2). The reduction in foot position in write,talk and
read compared to no phone condition represented a 48%, 20% and 11% change respectively.
Trail limb foot position was also significantly closer to the front step edge in write (42%
change) and talk (11% change) compared to read. The foot was also significantly closer in
write compared to talk condition (35% change).
There was a significant effect of phone condition on head angle (p <.01). At each instance
in the movement, the head was significantly more flexed in write compared to all other condi-
tions (Table 2). The head was also significantly more flexed in the read compared to no phone
and talk conditions at penultimate foot contact with the floor, final foot contact with the floor
and the instance of lead vertical toe clearance (Table 2).
There was a significant effect of phone condition on medial-lateral BVE (p <.001). There
was significantly greater medial-lateral variability in the write compared to all other conditions
(Table 2). The increase in medial-lateral BVE in write compared to no phone,talk and read
conditions represented a 137%, 88% and 121% change respectively.
Discussion
Pedestrians regularly engage with their mobile phone in a variety of ways whilst walking. Pre-
vious research has considered (separately) the impact of mobile phone use on visual search
behaviour and walking gait. However, to date, there is no published research collectively inves-
tigating how mobile phone use impacts visual search behaviour and adaptive gait, when
required to walk up to and safely negotiate a floor based obstacle. Findings from the current
study illustrate clear differences visual search behaviour and gait when using a mobile phone
compared to when not using a phone. Using a phone, compared to not using a phone, resulted
in looking less frequently and for less time at the surface height change, and negotiating the
surface height change in a manner consistent with adopting an increasingly cautious stepping
strategy. Key differences in visual search and gait were also observed dependent upon the man-
ner in which the phone was being used.
Fig 4. Lead vertical toe clearance when negotiating the surface height change under different phoneconditions
(group mean ±SE). Write condition is significantly different to no phone,talk and read conditions.
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Phone vs. no phone comparison
Results from the current study support previous findings highlighting that whilst walking,
compared to no phone being present, using a phone (either reading a text, typing a text or talk-
ing) results in increased trial time / reduced walking speed [3,12–14,16]. However, we extend
previous research to demonstrate that this is also evident when negotiating a surface height
change. The current research also shows that when using a phone, increased trial time (and by
implication reduced walking speed) was attributed to participants reducing both lead and trail
foot stride length (due to altered foot placement pre / post surface height crossing) and has
previously been shown to decrease as walking speed reduces e.g. [35] and lifting the feet higher
and slower when negotiating the edge of the surface height change (Table 2,Fig 4).
Previous research has attributed reductions in walking speed to a combination of dual-task
interference within the working memory [13], information processing ability [36], an attempt
to lower the cognitive demands of the primary (walking) task [37,38] or an effect of reduced
attention to walking [37]. Walking slower facilitates additional time to identify potential haz-
ards and plan/initiate a suitable response (i.e. walk round the object or step over the obstacle)
to minimise the risk of injury. However, it is important to recognise that walking slower when
engaged with the mobile phone may not necessarily reduce the risk of pedestrian injury if they
fail to ‘see’ the hazard in the first place, especially if the hazard suddenly appears in the envi-
ronment (i.e. pedestrian walks across the travel path). Indeed, in the write,read and talk condi-
tions, significantly fewer fixations were made to the environment (surface height change and
intended travel path, Fig 3A,Table 1) compared to no phone condition. If the pedestrian does
not re-orientate fixation towards the environment frequently enough, this may increase the
risk of accident as potential hazards will not be seen early enough to allow time to plan/initiate
a suitable avoidance response [17–19].
Interestingly though, the reduction in the relative number of fixations at the surface height
change ranged from 40–61% and the intended travel path ranged from 27–51% when using
the phone compared to no phone condition. Furthermore, in some instances in the current
study, in write,read, and talk conditions, throughout the entire trial participants did not look
directly at the surface height change. Despite this significant reduction in frequency (and
length) of fixation to the environment, participants were able to safely negotiate the environ-
ment/hazards placed in the travel path. Previous research has shown that effective and safe
travel is possible when a large proportion of time (~40%) is spent fixating at task irrelevant
objects [20] and without having to reorientate fixation to directly look at key areas/object of
interest. For example, the use of peripheral vision can be effective in safely stepping over an
obstacle [21] or up onto a step [22] and when using a mobile phone, navigate whilst cycling
[23] driving [24,25] and walking [14,16]. Indeed, Ahlstrom et al. [23] highlighted that whilst
cycling outdoor and engaging with a phone, individuals had fixations to the phone frequently
exceeding 5 seconds and in some instances reaching ~20 seconds. Despite these long periods
whereby no visual information was acquired from the road using the fovea (central vision),
cyclists did not crash. This led Ahlstrom et al. [23] to suggest that cyclists were able to effec-
tively rely on peripheral vision for guidance while their fovea was directed at the phone. In the
current study, no contacts with the surface height change were observed, indicating that con-
current visual information from the fovea may not be necessary to safely negotiate complex
terrain whilst engaging with a mobile phone. It is likely that participants use a combination of
central and peripheral vision to navigate their travel path when interacting with a mobile
phone.
A possible mechanism to adapt to the task when stepping up onto the surface height
change, compared to the no phone condition, were that participants lifted the lead and / or trail
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foot significantly higher over the edge of the surface height change and / or crossed the edge
with significantly slower horizontal foot velocity (Table 2,Fig 4). Increasing foot clearance
over the obstacle provides a greater margin for error, subsequently reducing the risk of con-
tacting the surface height change. Reducing crossing velocity minimises the risk of tripping if
contact is made with the surface height change edge, since any contact will have less effect on
perturbing balance [18]. These adaptations in the movement indicate a cautious stepping strat-
egy, which may be the result of two factors. The significant reduction in time spent looking at
the surface height change when using the mobile phone subsequently meant that participants
were unable to acquire a precise visual representation or accurately perceive the obstacle’s
characteristics prior to stepping up. Alternatively, participants were required to rely on infor-
mation from the peripheral visual field, which has previously been shown to provide poorer
resolution of fine detail (visual acuity, [39]) and spatial modulation sensitivity [40] compared
to central vision.
The increase in medial-lateral deviation when using the phone may indicate an increased
risk of injury compared to not using a phone. When walking up to and negotiating the surface
height change, participants demonstrated greater medial-lateral deviation in write compared
to the no phone condition (write was also significantly different to read and talk conditions,
Table 2). Increased medial-lateral deviation when walking poses implications for pedestrian
safety since deviating from the intended travel path increases the risk of colliding into oncom-
ing pedestrians and objects [3], which is a leading cause of mobile phone related pedestrian
injury [41].
Increased medial-lateral deviation in the write condition compared to the no phone (and
indeed read and talk conditions) is likely attributed to the associated changes in head flexion
and visual search behaviour. In the write condition, participants significantly increased head
flexion and looked down at the phone for longer and more frequently compared to all other
conditions (Tables 1and 2). The increased visual attention to the mobile phone resulted in
reduced fixation time at the intended travel path in the write condition compared to the no
phone (and read and talk) conditions. Because of this, participants may not have been able to
acquire sufficient visual information using the central visual field to plan direction of travel.
Alternatively, due to the position of the head (increased flexion) it is possible that the periph-
eral visual field was unable to acquire adequate visual information regarding path planning.
When using the mobile phone, compared to the no phone condition, the lead foot was posi-
tioned significantly closer to the front edge of the surface height change after stepping up onto
the upper level (ranging from 36–41% change, Table 2). Inappropriate foot placement when
stepping up to a new level will increase the risk of falling since the position of the foot deter-
mines the quality of the base of support for the weight-bearing phase whereby only that foot is
in contact with the ground [29]. However, in the current study, despite the foot being posi-
tioned closer to the front surface height change, the entire foot was always safely positioned on
the step (i.e. the heel was never over hanging the front edge of the step). This findings does not
suggest reduced stability / increased risk of falling, rather likely reflects the reduced stride
length observed (Table 2) with walking slower.
Within phone comparison
When engaging with the mobile phone, the greatest adaption in gait strategy was observed in
the write compared to the read and talk conditions. In the write condition, when stepping up
onto the surface height change, participants lifted their lead foot significantly higher over the
edge of the surface height change and crossed with a slower lead and trail foot velocity com-
pared to the read and talk conditions (Table 2,Fig 4). Participants also reduced their stride
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length, walked slower (increased trial length) and demonstrated greater medial-lateral devia-
tion. Adaptations in gait strategy was also observed in read compared to talk condition
(Table 2). Some of these adaptations have been discussed in the previous sub-section.
The differences observed in walking gait in the write and read compared to talk condition
may be attributed to the type of information relied on in the secondary task. Completing the
walking task would have predominantly relied on the visual-spatial resources of the working
memory. The read and write conditions would have also relied on the same visual-spatial
resources of the working memory, whereas the talk condition would have likely drawn on the
more distinct resource from the phonological loop. The greater changes in gait in the write
and read compared to talk condition may be attributed to the utilisation of the same working
memory resources as opposed to the sharing of working memory demands in the talk condi-
tion [42–45]. In the write condition, the added motor demands required for reading and writ-
ing the text message, in addition to the cognitive processes required for the communication
interchanges [26] may explain why write had the greatest effect on walking gait when com-
pared to read condition. Furthermore, writing a text may increase the visual attention
demands required to locate the keypad to type the text in addition to confirming (reading)
what has been written on the screen is correct [42]: which may explain why the number and
duration of fixations at the phone was greatest in this condition (Table 1).
In the write and read condition, throughout the trial a large proportion of visual attention
was directed towards the phone (~88±13% and ~61±22% in write and read conditions respec-
tively, Table 1). Whilst the proportion of visual attention being directed towards the phone
was greater in the read compared to talk and no phone conditions (where the phone was not
present in the visual field), this was significantly less than the write condition. The reduction in
time spent fixating at the phone in the read compared to write condition facilitated additional
time throughout the trial to fixate at the intended travel path direction (~6±10% and ~18±15%
in write and read conditions respectively, Table 1). The additional visual information acquired
from the intended travel path may explain why in the read condition, medial-lateral deviation
was significantly less than the write condition and similar compared to the no phone and talk
condition. Furthermore, with the head being less flexed (i.e. looking up more within the envi-
ronment) in the read compared to the write condition (Table 2), this facilitated the use of the
superior peripheral visual field to provide directional guidance via optic flow [46]; the complex
flow pattern of visual motion at the retina.
It is important to emphasise that the results from the current study do not provide a direct
indication of why mobile phone use has been reported to increase the risk of pedestrian related
accidents (e.g. [9]). Generally, the present findings actually indicate a more cautious strategy
when negotiating the surface height change located within the environment. It is therefore
likely that pedestrian related accidents when engaged with a mobile phone are attributed to
tasks that require greater attentional demands and thus provide a greater demand on working
memory; such as walking down the street where you are required to attend to numerous
potential hazards, crossing the road when you are required to attend to oncoming hazards (fel-
low pedestrians or cars) travelling at different velocities e.g. [27] or when the phone interaction
is increasingly demanding i.e. a particularly engaging/important conversation.
Limitations
The limitation with using the eye tracker is the assumption that people are attending to where
they are looking. Research has shown that it is possible to ‘shift’ our attention without moving
our eyes [47]; pedestrians may not be perceiving information from the precise location where
they are looking. However, with complex stimuli such as walking in the environment, research
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has shown that it is more efficient to move our eyes than move attention [48,49]. Conversely, it
is possible that participants were looking at key areas within the environment (e.g. surface
height change) but failing to perceive or process the obstacle’s characteristics. Indeed, Stavri-
nos et al., [27] identified that in a virtual street crossing task, when talking on the phone,
pedestrians appeared to be engaging in appropriate precautionary measures prior to crossing
the road (turning their head from side-to-side to inspect oncoming traffic), but were still
involved in significantly more virtual accidents (i.e. hit by a vehicle) compared to crossing
without a phone, which was hypothesised as a result of failure to detect or process critical
information pertaining to oncoming vehicles.
In the current study, participants completed the task in a stable environment. The results
from the current study suggest that pedestrian related accidents when using a mobile phone
are likely attributed to unexpected changes in the environment not being seen or reacted to by
the pedestrian. Indeed, an unexpected change in the environment should produce a reorienta-
tion in attention [50,51], however, if attention is currently focussed on the mobile phone or
distracted somewhere else [52], this reorientation in attention may either not occur, or occur
too late to avoid accident. In real world situations (i.e. outside of a laboratory testing environ-
ment) pedestrians face obstacles which they will have seen several steps in advance (e.g. a kerb
or step), to something they may not have seen or has suddenly appeared during the concurrent
step (e.g. another pedestrian changing direction and walking across their path). Our study
adopted a stable scenario whereby the obstacle and surface height change were presented sev-
eral steps in advance of the participant. This is similar to the approach used in other walking
based studies e.g. [13,14,37]. Future research should consider investigating the impact of
mobile phone use during a more dynamic environment as this would probably elicit a different
response in gait and visual search to that observed in the current study. However, this future
research direction should be considered separately to the findings reported in the present
study due to the extent this occurs in real world situations (i.e. obstacles rarely appear in the
travel path).
In the current study, participants negotiated the surface height change leading with either
the left or right limb. Previous research has demonstrated that gait asymmetry can influence
the kinematics of level walking and obstacle crossing e.g. [53,54]. In the present study, limb
dominance was not recorded from participants, resulting in being unable to aggregate the data
accounting for foot dominance when negotiating the surface height change. Whilst the impact
of gait asymmetry may act as a confounding influence within the present results, it is relevant
to note that gait asymmetry has predominantly been reported among older adults (the present
study used young healthy adults) due to muscle imbalance in the leg e.g. [55,56]. Furthermore,
the strength of the significance and reported percentage differences between mobile phone
conditions for lead and trail limb variables indicates that we can be confident of the findings.
Conclusion
The current study investigated how mobile phone use affects pedestrian’s visual search behav-
iour and gait when negotiating a floor based hazard in the travel path. Findings support our
initial hypotheses, highlighting that pedestrians altered their visual search behaviour and adap-
tive gait when using their phone compared to no phone being present. Furthermore, tasks
which required the individual to fixate predominantly at the phone’s screen had a greater effect
on pedestrians’ visual search behaviour and adaptive gait. Specifically, compared to the no
phone condition, adaptations in visual search behaviour and gait were observed when writing,
reading or talking on the mobile phone. The adaptations in gait when negotiating the surface
height change were consistent with participants adopting an increasingly cautious stepping
The impact of mobile phone use on where we look and how we walk when negotiating floor based obstacles
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strategy which may serve to reduce the risk of tripping / falling. Writing a text message whilst
walking resulted in the greatest adaptions in visual search and gait compared to reading a text
and talking on a mobile phone. Collectively, these findings indicate that mobile phone users
adapt their visual search behaviour and gait to incorporate mobile phone use in a safe manner
when negotiating static floor based obstacles.
Supporting information
S1 File. Raw gait and visual search data from each participant group in each test condition.
(ZIP)
Author Contributions
Conceptualization: MAT, KVP.
Data curation: MAT, HB, KT, KVP.
Formal analysis: MAT, HB, KT, IB, MJDT, KVP.
Investigation: MAT, HB, KT, KVP.
Methodology: MAT, KVP.
Project administration: MAT, HB, KT, KVP.
Resources: MAT, KVP.
Software: MAT, HB, KVP.
Supervision: MAT, KVP.
Validation: MAT, HB, KT, IB, MJDT, KVP.
Visualization: MAT, HB, KT, IB, MJDT, KVP.
Writing – original draft: MAT.
Writing – review & editing: MAT, HB, KT, IB, KJDT, KVP.
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