Traffic Safety Evaluation Of Arterials With Signal Coordination

Signal coordination has been popularly implemented at signalized intersections along the urban arterials.It can generate the green waves for vehicles to travel through the intersections with fewer or no stops.The state-of-the-art studies on signal coordination are mainly focused on the optimization of timing scheme in order to achieve the maximum operational efficiency.The impacts of signal coordination on the traffic safety,however,are largely overlooked in previous studies.Few studies are available to identify the change of the safety performance of arterials with the implement of signal coordination,which may vary due to the changing traffic flow patterns.In recent years,scholars have gradually paid attentions to the safety issues associated with the operation of coordinated arterials.The related studies identify the safety count changes in terms of crash frequency or traffic conflicts before and after the signal coordination of the coordinated arterials.Notwithstanding,it is still unclear how the signal coordination affects the traffic safety at intersections and along the arterials.Thus,the safety issues of signal coordination deserve an in-depth exploration.Therefore,the study aims to evaluate the safety impacts of signal coordination on the arterials in terms of spatial distribution of crashes,crash risk,and injury severity.Crash data(2000-2014)on six arterials in the city of Ann arbor,Michigan which are collected by Michigan Department of Transportation are retrieved for the safety evaluation.In the aspect of spatial distribution of crashes,kernel regression model is adopted to evaluate the safety performance of intersections before and after implementing the signal coordination strategy.By using this statistical method,the author identifies the non-linear relationship between crash frequency and the crash spatial location,and examines the discrepancy of crash spatial distributions between the coordination and non-coordination conditions at the disaggregated levels,such as time of day and crash types.A case study is presented with the use of Michigan crash data.The study finds that 1)crash distribution on arterials tends to be spatially disperse when the signal coordination is in operation;and 2)crash frequency on the road segments is increased with the use of signal coordination under the following conditions: non-peak hours,rear-end and side-swipe crashes,intersections with low speed limit,and both injury and property damage only crashes.The findings indicate that the operation of signal coordination does bring about safety concerns besides its operational benefits for the intersections.With respect to the crash risk,the study compares the crash propensities of various driving cohorts(measured by relative crash involvement ratio)on arterials with and without the signal coordination with quasi-induced exposure technique which has been well developed in estimating crash risks for driver-vehicle characteristics(i.e.,driver age,gender,and vehicle type).The significance of the relative crash risk is tested by using mixed-effect logistic model which considers both fixed and random effects of variables.Michigan traffic crash data are retrieved for the case study.The results indicate that 1)when signal coordination is implemented,young,male drivers,and pickups are associated with more crash responsibilities;2)crash propensities vary for different disaggregated situations,e.g.,young drivers may experience the rapid increase in crash risks during the peak hours;and 3)more hazardous actions(e.g.,failing to stop in assured clear distance)are witnessed for the high-risk driving cohorts on the coordinated arterials than non-coordinated ones.In terms of the crash injury severity,the study assesses the impacts of signal coordination on the injury severity with using Chi-square testing,and identifies the influencing factors by employing the random parameter ordered logit and generalized additive models.In the case study in Ann Arbor,Michigan,results indicate that 1)the signal coordination may decrease the probability of minor injury crashes but increase the frequency of crashes with more severe injuries;2)many factors(e.g.,the number of involved vehicles,female drivers,time of day,crash types,hazardous actions,and abnormal driving conditions)are found to contribute to the injury severity;and 3)the injury severities exhibit more spatial dependence on the coordinated arterials as opposed to those on the non-coordinated ones.In addition,the study aims to examine the contributing factors on injury severities by employing two-layer stacking machine learning model in which the first layer combines the random forest,neural network,and boosted classification tree methods,and the second layer is performed with the conventional logistic regression model.The results indicate that 1)the stacking model performs better than each individual model;and 2)the crash injury is prone to become more severe when it is a multiple-vehicle crash,head-on or angle crash,involved with abnormal driver's behavior(drinking and distraction),hazardous actions(disobeying traffic control devices and failing to yield),or occurring at midnight.The study highlights the importance of safety impact analysis of signal coordination.It can serve to provide the scientific basis to design and improve the signal control and management from a traffic-safety perspective.The findings will help to develop the effective countermeasures to reduce the crash risk and mitigate the injury severity,thus consequently improve the safety operation of arterials with the signal coordination.