Can headway reduction in fog be explained by impaired perception of relative motion?

Eye movement as a function to explore the effects of improved signs design and audio warning on drivers' behavior at STOP-sign-controlled grade crossings

Train-vehicle collisions at STOP-sign-controlled grade crossings attract many concerns in China and across the world. Researchers have demonstrated that the cost-effective approaches to improve grade crossing safety are the redesign of signs and pavement markings as well as the application of in-vehicle audio warning. However, the impacts of improved signs design and audio warning on drivers' visual performance have barely been discussed. This study explored the effects of improved signs design and audio warning on drivers' eye movement patterns and driving behavior at STOP-sign-controlled grade crossings, by conducting a driving simulator experiment. Three types of grade crossing scenarios: 1) the conventional signs design (Baseline), 2) improved signs design (PS), and 3) improved signs design and three-stage audio warning (PSW), were modeled in a driving simulation system and tested under a series of train TTC (no train, 4 s, 7 s, 10 s, 13 s) conditions. Foggy conditions and drivers' characteristics, i.e., gender and vocation were considered in the experiment design. Seven variables describing both drivers' fixation patterns and driving performance were collected and analyzed in this study, e.g., total fixation duration, distance to stop line at the first fixation, fixation transition probability, stop compliance, speed, maximum deceleration rate and minimum time-to-collision. Results revealed that the improved design of signs and the audio warning could prime drivers' expectation of the grade crossing in advance since drivers could drive at a lower speed, perceive signs timely, and conduct an earlier visual search for the train with these countermeasures. Besides, in PS and PSW scenarios, drivers attached more importance to the STOP sign, and they were more cautious in estimating the time-to-arrival of the train by repeatedly fixating on these two areas. The improvement in fixation performance of drivers in PS and PSW contributed to a more comfortable deceleration. Compared with no warning scenarios, higher compliance rates were observed with audio warning, especially with a short train TTC (4 s and 7 s). However, no significant difference was found between PS and Baseline, indicating the limited safety benefits of improved signs design. Minimum time-to-collision for those drivers who ignored the warning did not increase significantly in both PS and PSW. Additionally, heavy fog limited drivers' perception of signs and led to a later and shorter fixation. For gender effect, males had a lower fixation duration on the STOP sign and lower compliance rate than females. Moreover, female drivers could perceive the approaching train earlier than males, especially in PS and PSW. These findings suggested that the improved signs design and in-vehicle audio warning improved drivers' visual and behavioral performance and had the potential to enhance safety at STOP-sign-controlled grade crossings.

The foggy effect of egocentric distance in a nonverbal paradigm

Inaccurate egocentric distance and speed perception are two main explanations for the high accident rate associated with driving in foggy weather. The effect of foggy weather on speed has been well studied. However, its effect on egocentric distance perception is poorly understood. The paradigm for measuring perceived egocentric distance in previous studies was verbal estimation instead of a nonverbal paradigm. In the current research, a nonverbal paradigm, the visual matching task, was used. Our results from the nonverbal task revealed a robust foggy effect on egocentric distance. Observers overestimated the egocentric distance in foggy weather compared to in clear weather. The higher the concentration of fog, the more serious the overestimation. This effect of fog on egocentric distance was not limited to a certain distance range but was maintained in action space and vista space. Our findings confirm the foggy effect with a nonverbal paradigm and reveal that people may perceive egocentric distance more "accurately" in foggy weather than when it is measured with a verbal estimation task.

The development and validation of video-based measures of drivers' following distance and gap acceptance behaviours

The distance at which drivers follow other vehicles has been found to be linked to crash risk. Tailgating (i.e. driving at an unsafe following distance) is both endemic and a leading cause of rear-end crashes. Similarly, drivers' decisions about when to merge with a stream of traffic are likely to influence crash risk. Consistent with this, it has been shown that crashes are more common at intersections where drivers more frequently have to slow for vehicles pulling out into insufficient gaps. Therefore, the development of reliable and valid measures of both of these driving behaviours would facilitate further crash prevention research. Given the problems associated with assessing these behaviours during real driving, we developed new video-based measures. In our new following distance measure, participants view videos shot from the perspective of a driver who is following another vehicle at a range of distances across a variety of traffic environments. On each trial, participants report their own minimum comfortable following distance relative to the following distance depicted in the video. In our new test of gap acceptance behaviour, participants view a series of video clips and indicate when they would pull out into the approaching stream of traffic shown in each clip. The two new measures each yielded reliable data, and we found that young drivers made riskier choices than older drivers for both following distance and gap acceptance. These age-related differences are consistent with those found in observational studies of real driving, supporting the proposal that the new tests could potentially be used as proxies for these crash-related driving behaviours in both lab-based research and large-scale online studies.

Effect of Imitation Phenomenon on Two-lane Traffic Safety in Fog Weather

A neighboring lane’s vehicles are potentially important influence factors of traffic safety. In fog weather, drivers will automatically imitate the behaviors demonstrated by other vehicles in the neighboring lane. To illustrate the effect of the imitation phenomenon on traffic safety, this paper develops an extended two-lane car-following model in fog weather. Numerical simulations are carried out to study the effect of imitation on multiple-vehicle collision induced by a sudden stop, as well as perturbation propagation when a small perturbation is added to the uniform traffic flow. The results indicate that the number of collisions depends on the influence coefficient of neighboring lane’s vehicles, sensitivity, headway and initial velocity. Furthermore, the number of crumpled vehicles decreases when the imitation phenomenon is taken into account. In addition, lower vehicular velocity in the neighboring lane can reduce the magnitude of acceleration and fluctuation of headway. The perturbation can be absorbed under certain given conditions regarding the imitation phenomenon. Therefore, traffic safety can be improved by considering the effect of the imitation phenomenon on two-lane traffic flow in fog weather. The findings in this study can provide a theoretical reference for the development of multi-lane intermittent release measures in fog weather.

Analyzing the effect of fog weather conditions on driver lane-keeping performance using the SHRP2 naturalistic driving study data

INTRODUCTION

Driving in foggy weather conditions has been recognized as a major safety concern for many years. Driver behavior and performance can be negatively affected by foggy weather conditions due to the low visibility in fog. A number of previous studies focused on driver performance and behavior in simulated environments. However, very few studies have examined the impact of foggy weather conditions on specific driver behavior in naturalistic settings.

METHOD

This study utilized the second Strategic Highway Research Program (SHRP2) Naturalistic Driving Study (NDS) dataset to evaluate driver lane-keeping behavior in clear and foggy weather conditions. Preliminary descriptive analysis was conducted and a lane-keeping model was developed using the ordered logistic regression approach to achieve the study goals.

RESULTS

This study found that individual variables such as visibility, traffic conditions, lane change, driver marital status, and geometric characteristics, as well as some interaction terms (i.e., weather and gender, surface condition and driving experience, speed limit and mileage last year) significantly affect lane-keeping ability. An important finding of this study illustrated that affected visibility caused by foggy weather conditions decreases lane-keeping ability significantly. More specifically, drivers in affected visibility conditions showed 1.37 times higher Standard Deviation of Lane Position (SDLP) in comparison with drivers who were driving in unaffected visibility conditions.

CONCLUSIONS

These results provide a better understanding of driver lane-keeping behavior and driver perception of foggy weather conditions. Moreover, the results might be used to improve Lane Departure Warning (LDW) systems algorithm by allowing them to account for the effects of fog on visibility. Practical Applications: These results provide a better understanding of driver lane-keeping behavior and driver perception of foggy weather conditions. Moreover, the results might be used to improve Lane Departure Warning (LDW) systems algorithm by allowing them to account for the effects of fog on visibility.

Influence of Taillight Width on the Ability to Recognize Closing Speed, Closing Distance, and Closing versus Separating

Front-to-rear crashes account for a large number of fatal collisions in the United States. While many of these crashes might be related to driver error (i.e. following too closely, inattention to traffic ahead, etc.) a cluster of these crashes are likely to be related to the human visual system limitations of depth perception during motion. Observers with valid CDL and non-commercial licenses were shown two 4-second video clips showing a slower moving vehicle ahead, referred to as the lead vehicle. The lead vehicle was depicted at distances of 91 m (300 ft) to 457 m (1500 ft) while closing at 72 km/h (45 mph). The lead vehicle was depicted on an unilluminated two-lane highway at night to allow the taillights to be the salient stimulus. The lead vehicle had either the standard taillights with a width of 1.7 m (5.4 ft) or narrowed taillights that were 0.4 m (1.43 ft) apart. The order in which each clip was viewed was counterbalanced. Observers consistently believed the narrower taillight configuration was farther away despite the vehicles’ headlights being on, allowing the entire vehicle width to be seen at distances closer than 128 m (420 ft). Also, observers perceived the wider taillight vehicle to be closing faster when viewing at distances closer than 128 m (420 ft). Drivers with CDL licenses performed no better or worse than non-commercial drivers which supports the hypothesis that crashes involving a high-speed vehicle closing on a slow moving or stopped vehicle might be related to human limitations, rather than driving experience, inattention or careless behavior.

Automatic imitation of risky behavior: a study of simulated driving in China

OBJECTIVE

Imitation of risky behaviors among drivers is a potentially dangerous threat to driving safety but is infrequently discussed in the existing literature. To enrich the understanding of drivers' imitation behaviors on the road, 2 experiments were designed for a simulated traffic environment.

METHODS

Safe and risky behaviors were demonstrated by model vehicles separately in the 2 experiments, and imitation behaviors of the participants were observed and analyzed.

RESULTS

From experiment 1 it was found that the following distance of participants (measured in time headway) was affected by the distance demonstrated by other vehicles on the road. The influence was stronger when the speed was low, and the participants imitated both risky and safe behavior models. When the speed was high, the participants tended to only learn safe behaviors. In experiment 2, when approaching yellow lights, it was examined whether a driver's decision (pass or stop) would be affected by the behavior of another vehicle (the model vehicle), which was designed to either pass through or stop at the intersection. When the model vehicle ran the yellow light, 65 percent of the participants did the same, even though they were 30 m behind the model vehicle. In contrast, if the model vehicle stopped at the intersection, only 25 percent of the participants decided to pass.

CONCLUSIONS

It was found that both novice and experienced participants had the tendency to imitate what they saw but were rarely aware of the influence by other drivers in both scenarios.

Why do drivers maintain short headways in fog? A driving-simulator study evaluating feeling of risk and lateral control during automated and manual car following

Drivers in fog tend to maintain short headways, but the reasons behind this phenomenon are not well understood. This study evaluated the effect of headway on lateral control and feeling of risk in both foggy and clear conditions. Twenty-seven participants completed four sessions in a driving simulator: clear automated (CA), clear manual (CM), fog automated (FA) and fog manual (FM). In CM and FM, the drivers used the steering wheel, throttle and brake pedals. In CA and FA, a controller regulated the distance to the lead car, and the driver only had to steer. Drivers indicated how much risk they felt on a touchscreen. Consistent with our hypothesis, feeling of risk and steering activity were elevated when the lead car was not visible. These results might explain why drivers adopt short headways in fog. Practitioner Summary: Fog poses a serious road safety hazard. Our driving-simulator study provides the first experimental evidence to explain the role of risk-feeling and lateral control in headway reduction. These results are valuable for devising effective driver assistance and support systems.