Research On The Influence Of Posted Posted Speed Limit On Expressway Traffic Flow Characteristic Speed And Safety

Expressway,as a highway for high-speed vehicles,needs reasonable posted speed limit to ensure its safety and efficiency.However,reasonable posted speed limit has always been a challenge for posted speed limit policy makers and researchers due to the contradiction between traffic safety,operating speed and transportation efficiency.Considering only a single target to set the posted speed limit,it is easy to lead to unreasonable posted speed limit,with the driver’s expectations and road conditions,resulting in low social recognition of posted speed limit.Based on this,this paper studies the relationship between different characteristic speed and posted speed limit of expressway traffic flow through field speed collection.By comparing the historical accident data of expressways in China before and after posted speed limit increase,the impact of posted speed limit on traffic safety is analyzed.The relationship between restricted speed and traffic flow speed and traffic safety is found out,and the correlation model is established,which can not only clarify the overall impact of posted speed limit on traffic flow,but also provide some reference for the expressway management department to reasonably determine the posted speed limit scheme.On the basis of literature analysis at home and abroad,this paper takes the posted speed limit continuously changing section of Xi Kang expressway as the research object,collects the data of field traffic flow speed under different posted speed limit schemes and different traffic flow states,and puts forward the concept of characteristic speed of expressway traffic flow and its characteristic parameters.Through the method of regression analysis,using software such as SPSS and Origin,the linear relationship model between the posted speed limit and the characteristic speed parameters V85,V15 and V50 of the freeway traffic flow is established.The study shows that the characteristic velocity has a strong linear relationship with the posted speed limit under free flow.The change range of V85 and V50 is basically the same as the change range of posted speed limit value,but the change range of V15 characterizing heavy vehicles is only 50% of the change range of posted speed limit.In addition,in order to study the influence of road conditions on the model,this paper studies the relation between speed and posted speed limit of downhill section and tunnel section,and defines the applicable scope of the speed-characteristic speed model.The relationship between speed and posted speed limit and traffic safety is comprehensively analyzed,and it is considered that the theories of " speed lethal" and " speed difference lethal" are the causation theories of traffic safety at the micro or medium level,and are not applicable to the analysis of the impact of posted speed limit on safety from the macro perspective.Based on the adjustment of the national posted speed limit policy and the annual national highway accident data before and after the adjustment,the impact of posted speed limit changes on Chinese expressway safety is analyzed.It is found that the increase of posted speed limit will not cause significant changes in accident rate and severity of expressways in China.The research results have a certain reference value to guide the reasonable posted speed limit of the expressway.Among them,the effect model of the posted speed limit on the characteristic speed of the expressway is of great significance to enrich and develop the posted speed limit theory of the expressway.