Research On Traffic Conflict Prediction And Safety Evaluation Of Freeway Merging Areas

Merging areas are the entrance that vehicles pass through to enter freeway.Frequent speed change and lane change make the traffic operation and safety characteristics of merging areas are obviously different from basic segments,and the vehicles’ merging and weaving manoeuver is one of important reasons for affecting traffic safety of freeway.Merging area has thus become accident-prone location so that it’s necessary to investigate the safety of merging area.Given the practical issues of lacking complete and detailed crash data of merging area,the safety research on merging area of freeway was carried out based on traffic conflict technique in this paper.Taking merging areas of two-way-eight-lane freeway and two-way-four-lane freeway as research objects,the research scope of merging area is defined.On the basis of analyzing vehicles’ operation characteristics,and combined with the basic theory of traffic conflict technique,the definition and classification of traffic conflict of merging area are given,and the mechanism of generation is analyzed.Finally,post encroachment time(PET)is chosen as measure index for traffic conflict via comparative analysis of some common measure indexes.With survey data,the traffic flow characteristics of merging areas are analyzed,and then,the types and observational method of PET are introduced.By data fitting,the distribution of PETs of two-way-eight-lane and two-way-four-lane merging areas are determined as Gamma and Weibull distribution,respectively.Taking 15%,50% and 85% quantiles as thresholds for distinguishing severity of traffic conflict,the thresholds of serious,general and slight traffic conflict of two-way-eight-lane merging area are 1.23 s,2.50 s,4.44 s,respectively,and 1.56 s,3.39 s,5.80 s for two-way-four-lane merging area.Considering the relationship between traffic conflict and traffic flow parameter,traffic conflict is taken as dependent variable and traffic flow parameters are selected as independent variables,and on the basis of comparative analysis of passion distribution and negative binomial distribution,the prediction models of serious,general,slight and total traffic conflict of two-way-eight-lane and two-way-four-lane are built according to AIC and BIC criterion.And then,the prediction models are validated,and the results show that the prediction models built have high precisions on the whole with absolute values of relative errors less than 10%.The rates of serious,general and slight traffic conflict are selected as evaluation indicators,and combined with grey cluster theory,the safety evaluation method for merging area is proposed,and then,the safety levels of merging areas are determined,and the results show that the merging area of two-way-eight-lane freeway is safer than two-way-four-lane freeway totally,and the results are basically consistent with actual situation.The probability risk and speed risk are proposed as evaluation indicators and combined with one-way analysis of variance(one-way ANOVA),the safety of different lanes at merging areas is analyzed,and the results show that the probabilities of crash in outer lanes are higher than inner lanes at merging areas,and the lanes with higher probability risk have higher speed risk from the perspective of average value,additionally,the results of evaluation by using the two kinds of potential crash risks are not always consistent for the perspective of one-way ANOVA.