| Urban expressway has a dual function of highway and urban main road,and its role and status are beginning to be more and more prominent in road system.Though it has met with the traffic demands of medium-and-long distance as well as efficiency,a series of traffic problems such as traffic bottlenecks caused by traffic accidents,entrance ramps etc.,are emerging in an endless stream,which have seriously restricted the efficiency of road network.Based on cellular automata model,which has the merits of simple rules,easy expanding and high computationally efficiency,this paper models and simulates traffic bottlenecks in urban expressway accidents as well as entrance ramps,and then the effect of two kind of strategies of variable speed-limit control are analyzed with the aim of improving the road capacity.The main contents of this paper are as follows:1.The two-lane and three-lane models in the scene of traffic bottlenecks are proposed.Accident section is set to special lane-changing rules and updating rules according to the characteristics of accident bottleneck and nearby vehicles behaviors,which could simulate a series of alternating performance.In this paper,system flow,buffer flow and spatial-temporal evolution features are analyzed.The results suggest that,in the two-lane accident model,the road remained capacity proportion are both 37.36%when blocking the left and right single lane;while they are respectively 42.77%?43.26%and 39.08%when blocking the left,right or middle single lane in the three-lane accident model;at the spatial-temporal level,the upstream side of accident produces dense plugging zone while the downstream remain free flow state.2.The strategies of fixed speed limit and dynamic speed limit are presented.By adjusting the setting schemes of speed limit value as well as speed limit section,the changes of road capacity,point velocity and traffic flow time and space are simulated under different limited speed.The results showed as follows:In the fixed speed limit value strategy,the simple regulation of the speed limit can not improve the capacity of the road;Based on the average speed of all vehicles in dynamic speed limit strategy,the periodic renewal speed limit value has certain effect on improving road capacity.Besides,there will be pronounced effects with an isolation zone and refined limit zones,which can improve 11.37%-14.49%.Under the above two kinds of strategies,the average speed of all the vehicles at the accident location is gradually reduced,and the congestion degree is serious.These phenomena are consistent with the conclusion of traffic capacity.3.Ramp system is modeled by virtual ramp to study the influence of the entrance ramp on the main road traffic.The results suggest that the flow of upstream and downstream performed to increase firstly and keep stable when the entering probability is smaller,and the saturated flow at upstream and downstream are 0.883 veh/s and 1.072veh/s,at this time the ramp has been fully utilized.When the entering probability is greater upstream and downstream traffic is increased to stable,and the saturated flow at upstream and downstream are 0.77veh/s and 1.01 veh/s,while system capacity decreased by 6.83%.Beyond that the saturation flow rate of the ramp is less than that of the ramp.Applying dynamic variable speed limit strategy on the main road upstream,is founded to be able to significantly improve the saturated flow of the main road upstream and downstream capacity. |
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