Research On The Effect Of Posted Speed Limit On Expressway Traffic Flow And Effective Speed Control Methods

Speed limit is the core of speed management.It has an overall impact on traffic flow and has always been regarded as a key and difficult problem in the field of traffic engineering.Although developed countries in Europe and the United States have done a lot of research on this issue and mastered the law of the impact of speed restrictions on some aspects of traffic flow,some key problems still exist,and some results are not consistent with the actual situation of our country.By studying and analyzing the impact of speed limit on traffic flow,this dissertation aims to explore the quantitative relationship between speed limit and percentile speed,discreteness,transport efficiency,safety,fuel consumption and carbon emission.The determination method of reasonable speed limit and effective speed control measures are put forward.The relationship models between running speed(V₈₅),heavy vehicle speed(V₁₅),average traffic speed(V₅₀)and speed limit are constructed.By using the traffic data acquisition instrument with automatic identification of vehicle types,the vehicle speeds of several expressways in China under the conditions of different speed limit sections and different V/C ratio are collected,and the fitting equations of V₈₅,V₁₅ and V₅₀ percentile speed and speed limit are constructed by using Spss.The results show that V₈₅,V₁₅ and V₅₀all have a strong correlation with the speed limit,and the correlation between V₈₅ and the speed limit is the highest,and V₈₅ increases with the increase of the speed limit.It is found that the running speed(V₈₅)is usually lower than the speed limit,V₉₀is close to or below the speed limit.This finding is different from the conclusion of foreign research,which shows that the speed choice behavior of drivers in our country is greatly affected by speed limit.The effects of different speed limit on vehicle speed dispersion and the impact of different speed limit on expressway traffic efficiency are analyzed.Traffic flow speed data of the same traffic flow under different speed limits was collected.Statistical discrete indicators and description of traffic characteristics were used to evaluted the effects of different speed limits on vehicle speed dispersion.The analysis shows that with the increase of the speed limit,the distribution range of the traffic flow speed becomes wider and the speed becomes discrete.The higher the speed limit,the greater the speed dispersion.The main factor causing this gap is the distribution ratio of large and small cars in the speed distribution interval and the overspeed ratio of passenger cars.Under the conditions of free flow,relatively free flow,and steady flow,the passage efficiency of the road section increases with the increase of the speed limit.Under the congested flow state,the increase of the passage efficiency of the road section is not significant.For a given expressway,after the speed limit is increased,the improvement of the passage efficiency is mainly reflected by the increase in the average speed of the traffic flow.The impact of speed limit on expressways safety was analyzed in-depth in this dissertation.As the relationship between posted speed limit,driver speed selection and safety is complicated,in order to deeply analyze the impact of posted speed limit on highway safety,this dissertation explain the influence mechanism of speed limit on highway safety by constructing a framework of the relationship between speed limit and safety.This dissertation puts forward the theoretical point of view that the traffic accident on the highway is the speed incentive of the driver’s risk steady state,the defect of risk perception and the adverse consequence of speed difference coupling.Based on the energy model,by introducing the correction coefficient of speed limit law enforcement intensity,a model of the relationship between expressway accident mortality and speed limit is constructed.Based on the traffic accident historical data and highway transportation statistical data of our country in the past10 years,the accident rate,mortality rate and traffic flow related speed are statistically evaluated,and the relationship model between expressway accident rate,mortality rate and traffic flow speed is established.It is found that the accident rate and death rate of expressways have a high correlation with the average speed of the overall traffic flow and the average speed difference of heavy vehicles,but have a weak correlation with the average speed of the overall traffic flow.The research results verify the correctness of the theoretical viewpoint of highway accident causes proposed in this dissertation.A model of the effect of speed limit on fuel consumption and carbon emissions is constructed.The influence factors of vehicle fuel consumption and carbon emission on the highway are analyzed,and the fuel consumption data of two typical passenger and cargo vehicles under different longitudinal slopes and different speeds are collected through field experiments.Based on energy conservation and vehicle kinematics theory,the relationship models between fuel consumption,carbon emissions and speed of different vehicles are derived,and the relationship model between V₅₀ and speed limit constructed in this dissertation is adopted.To evaluate the impact of speed limit on fuel consumption and carbon emissions,it is found that for specific vehicle types and loads,vehicle fuel consumption or carbon emissions are mainly affected by vehicle speed.The speed limit and enforcement policy on the road will affect the driver’s choice of speed,and the impact of speed limit on vehicle fuel consumption or carbon emissions is holistic;with the increase of speed limit value,vehicle fuel consumption and carbon emissions increase accordingly.The increase of fuel consumption of small cars is higher than that of large cars,and higher than that of speed limits.The determination method of reasonable speed limit value and effective speed control strategy of expressway are put forward.This dissertation comprehensively analyzes the deep-seated causes of drivers’speeding,and puts forward the objective conditions and human factors leading to drivers’speeding behavior.In this dissertation,the theoretical basis and limitation of determining speed limit value based on design speed and running speed are analyzed,and the multi-objective decision-making method and model of expressway speed limit value under the condition of partial weight determination are put forward.An effective expressway speed control strategy based on driver adaptation mechanism is put forward.To sum up,this dissertation comprehensively constructs a quantitative model of highway speed limit on traffic safety and non-safety.It is revealed that speed limit has a certain impact on characteristic speed,speed discreteness,transport efficiency,safety,fuel consumption and carbon emissions.The research results provide good basic data for further research on speed limit.