Study On Traffic Capacity Of Freeway Sections Based On Simulation Technology

Capacity of road section is the basis for the planning, design and management of highways. However, latest study has indicated that the traditional approach for capacity calculation had been partly unreasonable. This paper performs a study with traffic simulation to overcome the limit of traditional method.Traffic models describe traffic operation, flow features and driving behavior with many independent parameters. Value of the parameters affect the simulation result greatly, therefore no simulation model shall be used or applied before strict calibration. A system for calibrating the model is developed in this paper, which includes the following procedures:identifying the calibration parameters, validating the range of value for the parameters, identification of the critical parameters, calibration based on Genetic Algorithm (GA), evaluation of the parameters and validation of the parameters. Approach has been developed for each procedure.Traffic survey was performed on typical freeway sections and the data collected was used to calibrate the parameters of VISSIM and CORSIM based on the approach as developed above. Critical parameters for calibration were identified from all the parameters that were provided by the two simulation software and the calibrated value was determined with Genetic Algorithm.The model was used to analyze the passenger car equivalent (PCE) of different vehicle types. It was found that the PCEs for different vehicle types varied when the operation conditions (lanes, speed limit, volume or density) change. Generally speaking, the PCEs determined by simulation are close to those in the Standard for passenger cars while those for small trucks are greater than those advised by the Standard.A capacity analysis was performed for the freeway section based on the calibrated model. The maximum theoretic capacity for freeway was figured out to be 2500pcu/h/ln based on the output Measures of Efficiencies (MOEs) and the criterion for maximum capacity. This value was a little greater than that specified in Highway Capacity Manual and Specification for Highway Alignment Design. Difference was also observed between the capacities as determined by simulation under given level of service and that in the Technical Standard of Highway Engineering (the Standard) in three aspects. First, the V/C under given level of service is smaller; second, the speed decreases slower with the increase of volume, third, capacity under level VI is smaller.