Traffic Flow Operational Characteristics And Control Methods At Roundabouts

As a special node in the road system,roundabout has drawn a lot of attentions of international transportation researchers and engineers,because of its high efficiency and low accident rate.Meanwhile,it has been applied widely in domestic urban road to meet the needs of traffic control and landmarks,such as Changchun city.However,with the rapid development of city and sharp increasment of vehicle parc,the traffic operation of unban roundabout is deteriorating in recent years.In order to improve this situation,some districts took the measure of removing or reconstructing roundabouts,in order to solve the traffic jam during short periods.However,due to lack of a clear understanding of traffic flow operational characteristics at roundabout,the above measures are not necessary to get a fundamental improvement of urban traffic system.Besides,aiming at ensuring the normal operation of traffic flow at roundabout,some traffic authorities set signals in the entries.But some signalized roundabout became the bottleneck of road network,due to lack of theory and method of signal timing at signalized roundabout.And it need the command of traffic police in peak hours.To solve the above problem,by the investigations of roundabouts in Changchun city,this paper emphasized to the traffic flow characteristics of roundabouts,such as headway.And it also studied method of modeling capacity of multi-lane roundabouts.Then from different perspectives,we proposed an tandem entry optimization method of roundabout based on pre-signals,and the optimizaiton method of roundabout based on upstream signalized intersection.In addition,an optimization method aiming at unbalanced flow at roundabout was also analyzed in this paper.The main research contents are as follows:(1)Analysis on traffic flow operational characteristics of roundabout.By investigating the headway data at large multi-lane roundabout in Changchun city,the distribution models were analyzed and fitted.Meanwhile,analyzing the impacts of with and without signals on the headway at roundabouts,the correlation tests were conducted.Then combining with the speed data,the speed distribution model was discussed briefly.By the use of VISSIM simulation,the impact of volume,left-turn ratio and heavy vehicles on operation efficiency were analyzed.In the last,considering setting slip lane at the entry,an comparison analysis was conducted from the whole perspective.(2)Capacity modeling at roundabouts.Entry capacity was analyzed from two different views:① Through verifing the limited priority phenomenon at roundabouts in medium-to-high volumes,and based on revised headway distribution model of circulatory lanes,the entry capacity of multi-lane roundabout was derived by gap-acceptance theory.②Based on conflict analysis theory,the coflict matrix of each traffic stream was built,then the entry capacity model of single-lane and double-lane roundabout were derived respectively using probability theory.Through comparison with some classical models,the applicability was verified.Moreover,the entry delay and queue length were discussed based on queue theory.Finally,different macrosopic scenarios were established by VISSIM,and the system capacity of single-lane and double-lane roundabout were built.(3)Optimization method of tandem entry of roundabout based on pre-signals.In saturation condition,in order to improve the traffic efficiency,the tandem entry is formed through setting signal before the stop line.Through analyzing the setting modes of pre-signals and combining with the geometry and traffic operations at roundabout,the corresponding signal control strategies were formulated.By setting optimizaiton objects and considering the cycle,split and the constraint conditions between main signals and pre-signals,the optimization control model was built.Finally,the feasibility of proposed method was verified.(4)The optimizing strategy of roundabout based on upstream signalized intersection control.For developing the effects of roundabouts in the unban network maximum,and aiming at congestion during peak hours,the jam at roundabout was dissipated indirectly through the adjustment of unpstream signalized intersections.Based on analyzing the relationships among different areas,the state index and correlation model of different areas were built.Then the mechanism of state identification and priority of control were formulated.According to above control strategy,the cycle,green time and offset adjustment method were put forward.Finally,the possibility of implementation of proposed strategy was also analyzed through a case.(5)The optimizaiton control method of unbalanced flow at roundabout.For the large delay of local entry due to the unbalanced flow,metering signal was set at the upstream approach,then the optimization model of control method was established.And the solve algorithm was built based on VISSIM simulaiton.Finally,through case study and sensitivity analyses,the feasibility of the above control method was verified.Finally,based on summing up the main achievements and progress of this paper,the main problem of this paper and the further research plans were put forward.