Over the past few decades,urban freeway congestion has been highly recognized as a serious and worsening traffic problem in the world.To relieve freeway congestion,several active traffic and demand management(ATDM) methods have been developed.Among them,variable speed limit(VSL) aims at regulating freeway mainline flow upstream to meet existing capacity and to harmonize vehicle speed.However,congestion may still be inevitable even with VSL implemented due to extremely high demand in actual practice.This study modified an existing VSL strategy by adding a new local constraint to suggest an achievable speed limit during the control period.As a queue is a product of the congestion phenomenon in freeway,the incentives of a queue build-up in the applied coordinated VSL control situation were analyzed.Considering a congestion occurrence(a queue build-up)characterized by a sudden and sharp speed drop,speed contours were utilized to demonstrate the congestion distribution over a whole freeway network in various scenarios.Finally,congestion distributions found in both VSL control and non-VS control situations for various scenarios were investigated to explore the impact of the applied coordinated VSL control on the congestion distribution.An authentic stretch of Whitemud Drive(WMD),an urban freeway corridor in Edmonton,Alberta,Canada,was employed to implement this modified coordinated VSL control strategy;and a calibrated micro-simulation VISSIM model(model functions) was applied as the substitute of the real-world traffic system to test the above mentioned performance.The exploration task in this study can lay the groundwork for future research on how to improve the presented VSL control strategy for achieving the congestion mitigation effect on freeway.
