Investigating driver reactions to movements of autonomous vehicle in permitted right turn through driving simulator experiments

Traffic safety has always been a hot topic for human-driven (HDV) and autonomous vehicles (AV) mixed flow. The conflict between permitted right-turn vehicles (PRT) and opposing through vehicles (TH) at signalized intersections (left-handed traffic) is extraordinarily critical. AVs with aggressive behaviors are able to accept short gap time without losing safety. However, such a turning maneuver may lead to dangerous feelings and cause unexpected reactions of approaching drivers. This study aims to investigate and model drivers’ reactions in TH movements to PRT AVs considering the trust degree of drivers to AVs. Questionnaire surveys and driving simulator experiments were conducted for 41 participants. Results reveal that the right turn timing of PRT AV will significantly influence drivers’ reactions. Basically, TH drivers will brake with a high probability under the situation of small expected post encroachment time (PET). It is also found that female drivers and drivers with low trust in AVs are more vigilant to PRT AVs than other drivers. Based on this finding a two-layer model for reproducing TH drivers’ reactions to PRT AVs is proposed. The first layer is to determine the braking decision and the second layer is to calculate the parameters of braking behaviors (brake lag, braking time, and speed drop). The significance and coefficients variables in these models proved that the trust in AV will influence drivers’ decisions and braking behaviors (brake lag and braking time). The more the drivers trust AVs, the smaller the expected PET to AVs they can accept for passing without braking, and the more gently they will brake (longer brake lag and shorter braking time) due to the cutting in of PRT AVs. This effect will become significant after drivers have experienced several interactions with AVs.     

Rear-view perception in driving: Distance information is privileged in the selection of safe gaps

Selecting a safe gap before merging into the traffic is a crucial driving skill that relies on images provided by rear-view mirrors or, recently, camera-monitor systems. When using these visual aids, some drivers select dangerously small gaps to cut in front of faster vehicles. They may do so because they base their decision either on information about distance or object size, or on miscalculated information about time-to-passage (TTP). Previous experiments have been unable to compare the role of TTP, speed, and distance information for drivers’ gap selection, as they did not investigate them in the same experimental regime. The present experiments seek to determine the perceptual variables that guide drivers’ rearward gap selection. Using short videos of an approaching vehicle filmed from three different camera heights (low, conventional, high), a total of 61 subjects either made gap safety decisions (Experiment I), or estimated the TTP, speed, and distance of an approaching vehicle (Experiment II). An effect of camera height was found for gap selection, TTP, and distance estimation, but not for speed estimation. For the high camera position, smaller gaps were selected as safe, TTP estimates were longer, and the distance to the approaching vehicle was perceived as farther. An opposite pattern was found for the low camera. Regression analyses suggested that distance is an important player. The subjects strongly relied on distance information when estimating TTP, and perceived distance dominated subjects’ gap selection. Thus, drivers seem to employ distance-based strategies when selecting safe gaps in rear-view mirrors or monitors.     

Can prosocial attitude reduce the risk behavior in simulated driving?

High traffic density may lead to more traffic accidents because of more frequent lane change and overtaking behaviors, but drivers with different characteristics may exhibit different driving behaviors. The present study explored the difference in driving behaviors between drivers with a high/low prosocial attitude under high/low traffic density. In this study, a 2 (high/low prosocial attitude) *2 (high/low traffic density) mixed design was used to investigate the interaction between prosocial attitude and traffic density on lane change and overtaking behavior. The implicit association test paradigm was used to measure prosocial attitude, and drivers were divided into two groups. Forty subjects were asked to complete simulated driving tasks under the two conditions of high and low traffic density, and driving behaviors were recorded by driving simulators. The results show that high traffic density leads to more lane change and overtaking behavior. Drivers with a high prosocial attitude have better driving performance under both high and low traffic density, but drivers with a low prosocial attitude maintain a smaller transverse distance from adjacent vehicles in high traffic density, which may increase risk. This study provides support for the selection, training and intervention of professional drivers.     

Gap acceptance at stop-controlled T-intersections in a simulated rural environment

A high proportion of road crashes occur at intersections: in Victoria, Australia, 15% of fatal crashes and 25% of serious injury crashes occur at T-intersections, with similar proportions occurring at cross intersections. Many of these crashes can be attributed to drivers’ inappropriate gap selection. The current study used a driving simulator to examine the influence of both the driver’s intended manoeuvre and the gap duration on gap acceptance behaviour at stop-controlled T-intersections. Drivers completed 18 gap acceptance trials, with manoeuvre (turning across traffic, merging with traffic) and gap duration (3–11 s) manipulated within-subjects. There was a trend whereby drivers accepted shorter gaps when turning across traffic compared to merging with traffic, which was significant at longer gaps (⩾9 s) but not at shorter, safety–critical gaps (⩽8 s). In addition, accepted lag times varied with manoeuvre. When merging with traffic drivers demonstrated longer accepted lag times, suggesting that turn strategies differ depending on traffic direction and intended manoeuvre. Overall the results suggest that the drivers’ intended manoeuvre influences gap acceptance, although gap duration remains the most influential factor. Implications of these findings for the development and design of intersection decision support systems are discussed.     

Social psychological models of choice behavior and drivers' left turns

ABSTRACT. The authors reanalyzed data from a simulated left-turn experiment (P. A. Hancock, J. K. Caird, S. Shekhar, & M. Vercruyssen, 1991) to test the adequacy of the nonlinear Gray-Tallman satisfaction balance model of choice behavior (L. N. Gray & I. Tallman, 1984) in predicting left turns. Participants (Hancock et al., 1991) were 40 experienced U.S. drivers who were exposed to simulated oncoming traffic; the size of the vehicle (motorcycle, compact vehicle, full-sized vehicle, delivery truck), its speed (10-70 mph, or 16-112 kmph), and the intervehicle time gap (3-9 s) varied. Hancock et al. (1991) measured (a) the likelihood of a left turn and (b) the occurrence of a collision. The probability of a left turn was greater for larger intervehicle time gaps and for oncoming smaller vehicles traveling at higher speeds. The Gray-Tallman (1984) model explained 69% of the variation in turning versus 57% for a linear regression model. In making decisions people tend to treat the values and costs affecting choices in a multiplicative, rather than linear, fashion. The Gray-Tallman model also has the potential for incorporating, both theoretically and mathematically, an unlimited range of potential values and costs that may influence left turn decisions.     

Social Psychological Models of Choice Behavior and Drivers' Left Turns

Abstract

The authors reanalyzed data from a simulated left-turn experiment (P. A. Hancock, J. K. Caird, S. Shekhar, & M. Vercruyssen, 1991) to test the adequacy of the nonlinear Gray-Tallman satisfaction balance model of choice behavior (L. N. Gray & I. Tallman, 1984) in predicting left turns. Participants (Hancock et al., 1991) were 40 experienced U.S. drivers who were exposed to simulated oncoming traffic; the size of the vehicle (motorcycle, compact vehicle, full-sized vehicle, delivery truck), its speed (10-70 mph, or 16-112 kmph), and the intervehicle time gap (3-9 s) varied. Hancock et al. (1991) measured (a) the likelihood of a left turn and (b) the occurrence of a collision. The probability of a left turn was greater for larger intervehicle time gaps and for oncoming smaller vehicles traveling at higher speeds. The Gray-Tallman (1984) model explained 69% of the variation in turning versus 57% for a linear regression model. In making decisions people tend to treat the values and costs affecting choices in a multiplicative, rather than linear, fashion. The Gray-Tallman model also has the potential for incorporating, both theoretically and mathematically, an unlimited range of potential values and costs that may influence left turn decisions.     

Analysis of driver’s decision distance and merging distance in work zone area based on parametric survival models: With the aid of a driving simulator experiment

Work zones affect traffic safety and efficiency by changing the road condition and drivers’ maneuver. Therefore, it is important to fully understand drivers’ merging behavior in work zone related areas. In this study, a model framework composed of decision-distance analysis and merging-distance analysis was proposed, which could describe both decision-making and lane-changing process of merging behavior. A road environment with work zone was developed based on a driving simulator, and six scenarios composed of two levels of traffic volume and three different lane-end sign’s locations were designed. Forty-two licensed participants, including 21 females (10 professional drivers vs. 11 normal drivers) and 21 males (15 professional drivers vs. 6 normal drivers) finally completed the experiment. Based on the experimental data, parametric survival models were established to analyze the effects of traffic sign location, traffic situation and driver characteristics on drivers’ decision distance and merging distance. The results showed that: (i) the lane-end sign’s location affected the decision point of lane changing and further affected the merging distance. However, the effect was weakened when the sign was placed far away from the work zone; (ii) merging distance in high traffic volume condition was shorter than that in low traffic volume condition; (iii) work zone posed greater challenges for female drivers as they merged later than males, and females were reluctant to adjust their merging distance according to different gap conditions. The findings shed some light on the future improvement of traffic design and management in work zones.     

Concept evaluation of intersection decision support (IDS) system interfaces to support drivers’ gap acceptance decisions at rural stop-controlled intersections

Rural stop-controlled intersections pose a crash risk for drivers turning or crossing the intersection from the minor road. In particular, elderly drivers are at the highest risk of a collision in this situation. Errors made during gap detection, perception and acceptance are the main factors that influence crashes at this type of intersection. This study investigated young (20–40 years) and old (55–75 years) drivers’ gap acceptance performance in simulated day and night driving conditions in a Baseline condition (STOP sign only) and four intersection decision support (IDS) conditions. The four IDS conditions were initial infrastructure-based design concepts that provided varying levels of dynamic information about traffic conditions on the major road to crossing minor-road drivers. Signs that provided detailed gap information (i.e., time-to-arrival values, warning levels for gaps) as well as advisory information about unsafe conditions resulted in the best performance among old and young drivers in comparison to signs that did not provide specific gap-related information (i.e., detected vehicles approaching, but not size of gap or safety of gap). Comprehension, acceptance and usability ratings of the IDS signs were also highest for signs providing detailed gap and advisory information on the same sign. Recommendations for further design and development of the IDS system interface based on driver performance and acceptance of the technology is discussed.     

Investigating the impact of a novel active gap metering signalization strategy on driver behavior at highway merging sections

A safe headway to the lead vehicle is important to reduce conflicts with merging vehicles from highway on-ramps. Previous research has outlined the advantage of gap metering strategies to yield sufficient space to merging vehicles and improve highway capacity during peak hours. However, prevailing gap metering systems fail to indicate the minimum required gap and leave it to the drivers’ judgment to adjust their headway. This paper proposes a new Active Gap Metering (AGM) signalization that helps outer lane drivers to adjust their headway to the lead vehicle when approaching highway ramps with incoming vehicles. This AGM signalization represents a combination of pavement markings and an innovative Variable Message Sign (VMS). The AGM system was tested alone and in combination with additional variable speed limits (VSL) in distinct environments of the Doha Expressway in the State of Qatar using a driving simulator. The driving behavior of 64 drivers was analyzed using repeated-measures ANOVA. The results showed that the AGM effectively influenced the drivers’ behavior on the right stream lane. Drivers did gradually increase the distance to the lead vehicle, which resulted in optimal headways to merging on-ramp vehicles. Most importantly, the minimum time-to-collision (TTC_min) to the merging vehicle was increased by an additional 1–1.5 s as compared to no treatment. The proposed AGM signalization can, therefore, be considered by policymakers to influence drivers’ headways at critical merging sections.     

Using a multi-user driving simulator system to explore the patterns of vehicle fleet rear-end collisions occurrence under different foggy conditions and speed limits

Vehicle fleet rear-end collisions (FRECs) are an extremely fatal type of traffic collisions on freeway and they usually occur in foggy weather. This study aimed to explore the patterns of vehicle fleet rear-end collisions occurrence under different foggy conditions and speed limits on freeway. A multi-user driving simulator system was used to conduct the experiment and the driving behavior data were collected from eight participants. The experimental results showed that as the fog density increased, the length of vehicle fleet decreased significantly, and drivers tended to keep a more stable car-following distance. The fog weather and short vehicle gap prompted drivers to react faster and brake harder in respond to the leading vehicle’s brake. In spite of the compensational behaviors, more FRECs were observed under heavy fog condition. Lowering speed limit can significantly reduce the FRECs under foggy conditions. As the speed limits reduced, drivers’ brake response time and speed variance significantly reduced. The study also found that drivers’ brake response time was negatively correlated with their positions in the fleet. Drivers in the front positions of the fleet had a longer response time than drivers in the back positions and thus were more likely to encounter collisions. The study generated a better understanding of drivers’ behavioral pattern in a vehicle fleet and the patterns of vehicle fleet rear-end collisions occurrence. The findings also shed lights on the design of driver assistance system for complex driving situations such as freeway driving under adverse weather.     

The nudging effect of social norms on drivers’ yielding behaviour when turning corners

Drivers who do not give way to pedestrians are among the main causes of traffic crashes of motor vehicles and pedestrians in China. There is an urgent need to explore effective methods for improving drivers' yielding behaviour. This study used eight kinds of social norm slogans to nudge drivers’ yielding behaviour and explored the influence of pedestrian characteristics and situational factors on drivers’ decision-making regarding giving way when turning right. By analysing 254 valid questionnaires, it was found that compared with static norms, injunctive norms and dynamic + static norms, positively worded dynamic norms have a better nudging effect on drivers' yielding behaviour. The results of the conjoint analysis showed that social norms have the greatest impact on drivers' decisions to yield (32.28%), followed by whether pedestrians comply with traffic rules (23.33%) and the age of pedestrians (18.40%). This study expands the application of social norms and provides a new perspective to promote positive behaviour among drivers.     

A simulator study of factors influencing drivers’ behavior at traffic lights

Drivers’ reactions to changing traffic lights have an impact on safety at intersections. We examined the influence of transient factors – more specifically time pressure and social context, both conducive to traffic-light violation – on behavior behind the wheel when a traffic light changes. We carried out an experiment on a driving simulator. The participants were 94 car drivers (53 males) with a mean age of 21.7 years. They drove under time pressure vs. no time pressure. At several intersections the participants were alone (no other drivers present), whereas at several other intersections they were behind a line-up of vehicles, the last of which ran the yellow light (other drivers present). As expected, time pressure and social context (presence of other drivers) increased participants’ risky behaviors while approaching, and going through traffic lights, as well as undesirable rapid accelerations when the signal changes to green. The effect of time pressure on yellow-light running was not mediated by approach speed, which showed that participants in a hurry were likely to run lights intentionally. The results are interpreted in view of proposing effective measures for reducing yellow-light running and rapid accelerations at traffic lights.     

Optimizing guidance signage system to improve drivers’ lane-changing behavior at the expressway toll plaza

Although ETC (Electronic Toll Collection) has been widely implemented in vehicles in China in recent years, the channel guidance signage system on expressways has not been updated accordingly. Late lane-changing occurs when drivers are confused about the correct channel to enter, leading to increased crash risks and traffic congestion in front of the toll. The paper aims to optimize the current guidance signage systems, given the high proportion of ETC vehicles on Chinese expressways, and evaluate its effects on the drivers’ lane-changing behavior when passing through the expressway toll channels. A driving simulator experiment was conducted to test four scenarios: Original sign plan, Partial Manual Toll Collection (MTC) sign plan, Complete MTC sign plan and Complete MTC sign with Voice warning plan. Forty participants with a valid driver's license completed the four scenarios, and their behavior performances (e.g., decision-making of lane-changing, response time, average speed and deceleration) in the main lane in front of the toll booth were analyzed. The results showed that compared to the Original sign plan and Partial MTC sign plans, the Complete MTC sign plans (with and without voice warning) played a significant role in improving the MTC vehicle drivers’ lane-changing behaviors. The improvement included earlier initiated lane-changing, shortened response time, lower deceleration rate and extended lane-changing duration distance. The findings of this study have important implications for expressway designers and relevant management departments to optimize the current guidance signage and enhance traffic safety and efficiency at the toll plaza.     

Measurements of gaze movements while driving

In this study, gaze movements of drivers driving through an intersection were investigated. Gaze movements of drivers in large vehicles were compared with those in small vehicles. There were both similarities and differences in the visual search behaviors of drivers of large and small vehicles. The two groups were similar in that, when approaching an intersection, drivers made repeated saccadic gaze movements; after entering the intersection, saccadic gaze movements were directed ahead in the direction of turning. Differences arose in the frequency and distribution of gaze movements. The number of gaze movements was significantly greater in drivers of large vehicles. The distribution of gaze movements in driving a large vehicle showed a peak at the point 50 to 60 degrees to the right and left of the median plane of the driver. The distribution of gaze movements of drivers of small vehicles showed no peak across the visual field.     

Impact on driver behaviour of guardrails of different height in horizontal-vertical coordinated road scenarios with a limited available sight distance

Drivers consider traffic barriers (e.g., guardrails) a protection system, a hard obstacle and a sight obstruction. Hence, the possibility of using containment level barriers which are higher and superior than the minimum required by current standards should be carefully evaluated. Moreover, research investigations into their impact on driver behaviour should be designed so as to distinguish between the effects associated with each of the three roles cited above.This driving simulation study investigates how drivers adapt their longitudinal and transversal behaviour when negotiating curves with guardrails of different heights on horizontal-vertical coordinated two–lane rural road settings, with consideration given solely to the sight obstruction effect of the guardrails. Fifty-four participants drove four out of the eighteen possible scenarios obtained when the same horizontal alignment is combined with three vertical profiles with three inner roadside treatments (no guardrails, 0.75 m two–wave and 0.95 m three-wave guardrails) and the two driving directions.Research outcomes confirm that guardrail height has a significant impact on lateral and longitudinal behaviour. With the minimum standard, i.e., the minimum height, drivers stay closer to the roadside, while higher guardrails result in drivers increasing their lateral distance. Speeds are influenced by the interaction between the guardrail and other geometric and human factors. Male and female drivers adapt differently to the limitation in the available sight distance caused by the guardrail: males increase their speed, adopting a more aggressive behaviour than females. Important safety implications due to the higher speeds and wider trajectories have to be considered at the design stage.     

Passing behavior on two-lane highways

Two-lane highways make up a substantial proportion of the road network in most of the world. Passing is among the most significant driving behaviors on two-lane highways. It substantially impacts the highway performance. Despite the importance of the problem, few studies attempted to model passing behavior. In this research, a model that attempts to capture both drivers’ desire to pass and their gap acceptance decisions to complete a desired passing maneuver is developed and estimated using data on passing maneuvers collected with a driving simulator. Sixteen different scenarios were used in the experiment in order to capture the impact of factors related to the various vehicles involved, the road geometry and the driver characteristics in the model.A passing behavior model is developed that includes choices in two levels: the desire to pass and the decision whether or not to accept an available passing gap. The probability to complete a passing maneuver is modeled as the product of the probabilities of a positive decision on both these choices. The estimation results show that modeling the drivers’ desire to pass the vehicle in front has a statistically significant contribution in explaining their passing behavior. The two sub-models incorporate variables that capture the impact of the attributes of the specific passing gap that the driver evaluates and the relevant vehicles, the geometric characteristics of the road section and the driver characteristics and account for unobserved heterogeneity in the driver population.     

Exploring the effects of the location of the lane-end sign and traffic volume on multistage lane-changing behaviors in work zone areas: A driving simulator-based study

The lane-changing behavior in work zone areas has special features than a regular lane change as the former is usually compulsively motivated involving complicated cognitive processes with drivers’ perception of work-zone control devices. Toward this end, this study conducted a driving simulator-based experiment to understand the effects of lane-end sign distance and traffic volume on driving behaviors. A conceptual model was also proposed to partition the whole lane-changing process into three stages, i.e. the perception, preparation and action stages, reflecting different cognitive and manipulative activities of drivers. In addition to the lane-end sign distance and traffic volume, gender and profession of drivers were adopted as covariates. In this experiment, a complete combination of lane-end sign distance and traffic volume served as treatments. The results verify the impacts of those factors on driving behaviors in and across different stages. For example, the location of the lane-end sign had a profound influence on drivers' perception of the imminent work-zone situation, but the influence continued to diminish in the following two stages. On the other hand, male or taxi drivers tended to act earlier than female or regular drivers respectively, for all the three stages. According to the analysis, several practical implications were also provided. In specific, the lane-end sign is recommended to be installed 500 m upstream to the lane dropping point of work zones. It is a pioneer study toward investigating multistage driving behaviors in work zone areas, which is expected to provide references and guidance for the design of traffic control devices and other driving simulator-based studies.     

Research on the transition marking between the broken line and solid line of expressway

Drivers often cross the solid line unintentionally during the lane-changing, when the sight distance is blocked by the vehicle ahead. There are significant potential risks to change lanes in solid line area, which would cause traffic flow disruption and traffic safety hazards. If the drivers with limited sight distance can be reminded in advance that the lane marking ahead is a solid line, many drivers will refrain from changing lanes in the restricted area, therefore, it is important to help the drivers with limited sight distance to obtain the information of lane marking ahead. There is limited research on this specific issue presently, nor any relevant regulations or manuals available in China. This paper puts forward a novel concept of transition marking, and a creative approach by adding the transition marking between the broken line and the solid line. The transition marking is considered an information carrier to predict the pass-through right of the road ahead. We have studied the form and length of the transition marking and evaluated its effectiveness through the driving simulation experiment. The form analysis shows that the white mutation broken line has better visibility and legibility. The transition marking length is shown to be closely related to the vehicle speed and distance. The simulation results indicate that the transition marking length is negatively correlated with vehicle distance and positively correlated with speed. Based on the simulation results and detailed statistical analysis, the reference value of transition marking length is proposed. The effectiveness evaluation demonstrates that the transition marking reduces the probability of solid-line crossing during lane-changing dramatically by 92%.     

Manual drivers’ experience and driving behavior in repeated interactions with automated Level 3 vehicles in mixed traffic on the highway

In the near future, conditionally automated vehicles (CAVs; SAE Level 3) will travel alongside manual drivers (≤ SAE level 2) in mixed traffic on the highway. It is yet unclear how manual drivers will react to these vehicles beyond first contact when they interact repeatedly with multiple CAVs on longer highway sections or even during entire highway trips. In a driving simulator study, we investigated the subjective experience and behavioral reactions of N = 51 manual drivers aged 22 to 74 years (M = 41.5 years, SD = 18.1, 22 female) to driving in mixed traffic in repeated interactions with first-generation Level 3 vehicles on four highway sections (each 35 km long), each of which included three typical speed limits (80 km/h, 100 km/h, 130 km/h) on German highways. Moreover, the highway sections differed regarding the penetration rate of CAVs in mixed traffic (within-subjects factor; 0%, 25%, 50%, 75%). The drivers were assigned to one of three experimental groups, in which the CAVs differed regarding their external marking, (1) status eHMI, (2) no eHMI, and (3) a control group without information about the mixed traffic. After each highway section, drivers rated perceived safety, comfort, and perceived efficiency. Drivers were also asked to estimate the penetration rate of CAVs on the previous highway section. In addition, we analyzed drivers’ average speed and their minimum time headways to lead vehicles for each speed zone (80 km/h, 100 km/h, 130 km/h) as well as the percentage of safety critical interactions with lead vehicles (< 1 s time headway). Results showed that manual drivers experienced driving in mixed traffic, on average, as more uncomfortable, less safe and less efficient than driving in manual traffic, but not as dangerous. A status eHMI helps manual drivers identify CAVs in mixed traffic, but the eHMI had no effect on manual drivers’ subjective ratings or driving behavior. Starting at a level of 25% Level 3 vehicles in mixed traffic, participants' average speed decreased significantly. At the same time, the percentage of safety critical interactions with lead vehicles increased with an increasing penetration rate of CAVs. Accordingly, additional measures may be necessary in order to at least keep the existing safety level of driving on the highway.     

Driving around bends with or without shoulders: The influence of bend direction

Paved shoulders have long been used to create “forgiving” roads where drivers can maintain control of their vehicles even when as they drift out of the lane. While the safety benefits of shoulders have been well documented, their effects on driver behavior around curves have scarcely been examined. The purpose of this paper is to fill this gap by assessing whether the addition of shoulders affects driver behavior differently as a function of bend direction. Driver behavior in a driving simulator was analyzed on left and right curves of two-lane rural roads in the presence and absence of 0.75-m and 1.25-m shoulders. The results demonstrated significant changes in drivers’ lateral control when shoulders were provided. In the absence of oncoming traffic, the shoulders caused participants to deviate more toward the inner lane edge at curve entry, at the apex and at the innermost position on right bends but not left ones. In the presence of oncoming traffic, this also occurred at the apex and the innermost position, leading participants to spend more time off the lane on right curves. Participants did not slow down in either traffic condition to compensate for steering farther inside, thereby increasing the risk of lane departure on right curves equipped with shoulders. These findings highlight the direction-specific influence of shoulders on a driver’s steering control when driving around bends. They provide arguments supporting the idea that drivers view paved shoulders as a new field of safe travel on right curves. Recommendations are made to encourage drivers to keep their vehicle within the lane on right bends and to prevent potential interference with cyclists when a shoulder is present.