Study On The Road Safety Evaluation Of Suburban Arterials Based On Traffic Conflict Techniques

The road safety status on suburban arterials is not optimistic,which makes it become a key area for safety improvement.Traditional road safety study is mainly based on reported road accidents data which has quality problems.Traffic conflict technique has gradually attracted more and more attention.At present,the amount of systematic research on suburban arterials safety based on traffic conflict technique is obvious insufficient,which contracts with the urgent demand for safety management and improvement of this type of road facility.Therefore,based on the traffic conflict technique,exploring the road safety hazards and the contributing factors so as to make evaluation of safety level is of great significance for safety management and improvement of suburban arterials.Based on theoretical fundamentals of suburban arterial traffic conflict study,this dissertation analyzed the features,rules of occurrence and internal mechanism of traffic conflict through traffic conflict data collection and analysis.Conflict-based safety performance functions(SPFs)were also developed for the purpose of investigating the relationship between traffic conflict and safety,as well as establishing conflict-based suburban arterial safety evaluation methodology.Firstly,on the basis of suburban arterial roadway geometry,traffic operation and surrounding environment analysis,the basic definition of suburban arterial traffic conflict was discussed and the component of its formation was summarized.In addition,traffic conflicts were classified into different types,and the main features and causal mechanism of each type were explored.Traffic conflict indicators and severity stratification threshold were subsequently determined.Furthermore,the influencing factors and effects of traffic conflict were preliminarily investigated in order to provide references for Safety Performance Function(SPF)development.Secondly,on the basis of fundamental principles of SPF,some key issues were addressed in the development of conflict-based SPFs for suburban arterials which include model form,variable selection,parameter estimation,model performance assessment and comparison.Groups of SPFs were developed in this study: basic models(Poisson-Gamma and PoissonLognormal regression model),spatial model(Gaussian conditional auto-regressive model),models accounting for spatial clusters(random intercept model,random parameters model),and multivariate model taking into account the correlations among different levels of conflict severity.The modeling results indicate that suburban arterials traffic conflicts data exhibit evident over-dispersion,spatial correlation and multivariate nature.Analysis result of parameter values show that the frequency of traffic conflicts on suburban arterials are positively correlated with traffic volume,road segment length,proportion of motorcyclists and trucks in the traffic flow,presence of access driveways and number of lanes.Furthermore,from the viewpoint of conflict severity,it is revealed that exposure variables(i.e.,traffic volume and segment length)have stronger correlation with normal conflicts than severe conflicts,whereas other explanatory variables have stronger correlation with severe conflicts than normal conflicts.These findings indicate that increase in the proportion of motorcyclists and trucks,number of lanes and access points could more easily lead to increase in severe traffic conflicts.Afterwards,this dissertation has conducted research on conflict-based micro-level safety evaluation of suburban arterials.Techniques of hazardous location identification,ranking criteria,and effectiveness quantitative evaluation of safety improvement countermeasures were investigated.Empirical Bayes and full Bayes method were employed in the hazardous location identification.Ranking criteria and rules were put forward with respect to risk ratio,potential for safety improvement.Effectiveness evaluation of safety improvement countermeasures was introduced as Before-and-After study which combines SPFs and Odds Ratio calculation in order to quantitively evaluate the safety level enhancement.Finally,case study has undertaken on suburban arterials in Xi’an,China for the purpose of verifying aforementioned methodology of conflict-based safety evaluation.Specifically,the hazardous road segments were first identified and ranked.Road safety problems for these‘hotspots’ were then diagnosed and remedies were suggested accordingly.The effectiveness of these remedies was quantitively evaluated with combination of computer simulation technique.To sum up,this dissertation has proposed optional research ideas on suburban arterials safety study and accumulated research experience and outputs.At the same time,it could also provide references for suburban arterials safety management and administration.