Research On The Method Of Setting Speed Limits On Freeway And Urban Expressway

Freeway and urban expressway is the most important component part of road network, and how to realize its function of efficiency, safety, economy and comfort is always one of the key subjects for all circles in the society, especially the transportation industry. In order to guarantee realizing above function of freeway and urban expressway, the method of setting speed limit is usually adopted to manage freeway and urban expressway. Practice has proved that reasonable speed limit can promote raising the speed, safety, economy and comfortable level of freeway and urban expressway, however, inappropriate speed limit will be the opposite. Now, being restricted to financing, technology, systems and conception, the management level of freeway and urban expressway has been generally low and speed limit standards setting still lacks theory study results as technical support in our country. Aiming at analyzing systemly on various influence factors of freeway and expresswaon on speed and taking efficiency, safety, economy and comfort as goals of the system, this paper studied the method of setting speed limit that embodies system's dynamic characteristic of freeway and urban expressway to provide gist for speed management of freeway and urban expressway.Using methods of traffic surveys, statistic analysis, regression analysis, mathematics modeling and solving, theory derivation and so on, this paper studied the method of setting maximum and minimum speed limit, the problem of speed limit under adverse weather conditions and implement methods of speed limit.According to the alignment conditions of freeway and urban expressway, the definition conditions of at-grade straight line section, curve section and grade section were determined and traffic data collection plans were worked out. Criteria was given for the classification of vehicle types and calculation method of oversize vehicle proportion. Using statistics methods and regression analysis methods, the paper analyzed speed characteristic of different lanes and vehicle types on freeway and urban expressway under different alignment conditions respectively. The paper studied the relationship between traffic volume and traffic composition and percentile speed, and compared speed characteristics of freeway and urban expressways under adverse weather and sunny weather.Using the ideas of multi-objective optimization, the paper brought forward the speed limit method based on operation efficiency, safety, economy and comfort of expressway and urban expressway. The maximum speed limit value under the conditions of at-grade straight lines section, free traffic flow, and good pavement and weather is taken as fiducial value. By mathematics modeling and regression analysis, the time-cost function and fuel consumption cost function were established respectively, and quantification relationships was analyzed between speed and traffic accident severity and driving comfort on freeway and urban expressway. Then, taking minimizing the sum of time cost and fuel consumption cost as objective function and taking accident fatality rate and comfort that meet corresponding allowed range as constraint conditions, the fiducial value of the maximum speed limit calculation model was established based on minimum generalized running cost of vehicles on freeway and urban expreesway.Based on model parameters calibration, the paper adopted mathematics method of extremum solution to solve fiducial values of maximum speed limit calculation model and gave fiducial values of maximum speed limit according to different design speed and different vehicle types on at-grade straight lines section of freeway and urban expreesway. Using regression analysis method, the relationship model between alignment indexes, traffic flow parameter and operation speed of different vehicle types was established. Correction coefficients and corresponding maximum speed limt value of horizontal curve radius, grade and slope length, the number of lanes, traffic volume, traffic composition and pavement conditions were given.The paper brought forward the method of setting minimum speed limit based on speed dicrete degree constrain and economic sppeed. The concept and calculation method of speed dicrete degree was given and the relationship model was established between speed disctete degree and traffic accident rate by regression analysis method. Accepted value of speed disctete degree was determined refering related literatures, and constraint condition of minimum speed limit based on safety was given. The relationship model was established between fuel consumption and speed by regression analysis method and economical speeds under different grades were given. Taking minimal fuel consumption as objective function of the minimum speed limit, the minimum speed limit value was given of freeway and urban expressway according to the principle of maximizing operation efficiency.The paper analyzed the affecting mechanism of adverse weather conditions on raod trafic safety such as rain, snow, fog et al. Using the method of mathematics formulas derivation, calculation formula of maximum speed limit value was presented based on the safe distance and the suggestions of corresponding speed limit standards were given. Then, based on analyzing merits and demerits and applicability of speed limit of different lanes and different vehicle types comprehensively, the paper brought forward implement methods of speed limit of different vehicle types and different lanes according to different lane numbers. The concept of speed limit zone and continuity index of speed limit was given. Taking the continuity index of speed limit being less than 0.85 as dividing basis of speed limits zone, division results of speed limit zone were given. Then, the paper brought forward the method of setting speed limit at ramp exit of freeway and urban expressway based on safety speed, and gave suggestion values of maximum speed limit.