Simulation research and implemented effect analysis of variable speed limits on freeway

The congestion on freeway can be divided into demand driven congestion which is due to the increase of traffic volume and supply driven congestion which is due to the road´s geometric condition or traffic accident. The computer simulation is carried out in view of the cause of congestion and several factors that lead to the instability of freeway traffic flow such as small time headways, large speed variance, and frequent disturbances. Many kinds of scenarios of the variable speed limits system that can control the freeway traffic flow effectively are simulated. The simulation results indicate that the VSL benefits are obvious when the traffic volume is equal to or greater than 2800 veh/h (double lane) while resolving a demand driven congestion, which is shown as the increase of the served traffic volume, saving the travel time and reducing speed deviation. The results also indicate that it can reduce the queue time and stops, avoid happening of congestion and save travel time when the traffic volume is equal to or greater than 2000 veh/h (double lane) while resolving the supply driven congestion. However, the VSL acquit its limits when the traffic volume is greater than the traffic capability, it only delay the occurrence of congestion and do not avoid that, so VSL needs to cooperate with other measures such as RM (ramp metering) to control the traffic flow when the traffic is high. The simulation of variable speed limits system on freeway and the result analysis can offer the basis for implementing VSL, and can offer basic data for the social and economic impact evaluation of VSL.