An Analysis of Traffic Chaos on Urban Expressways under Variable Speed Limits

Variable Speed Limit (VSL)strategies have been used to manage traffic operation in urban expressway systems.To develop a reasonable speed limit values,the characteristics of traffic flow and speed should be taken into con-sideration.In this paper,traffic chaos on urban expressways is analyzed due to speed variation resulting from using VSLs. The ratio of vehicles complying with VSL in flow,p ,is applied as the key parameter to analyze the speed characteristics. A section of 3 rd Ring Expressway in Beijing is simulated using Vissim.The C-C method is used to reconstruct the phase space based on the speed time series data collected from Vissim scenarios.Small Data Set method is used to identify the traffic chaos by calculating the largest Lyapunov exponentsλmax .Traffic operations were compared between the scenarios with and without the implementation of VSLs.Analysis results showed thatλmax of speed time series is positive without VSL,andλmax of speed time series is also positive at the location 200 m downstream of VSL sign and a quadratic regres-sion model is found to fit betweenλmax and p .The minimum value ofλmax reached 0 when p =76.8%.The results of this paper can be useful to support the decision-making of the speed limits and implementation of the VSL strategies.