Online evaluation of an integrated control strategy at on-ramp bottleneck for urban expressways in Shanghai

Variable speed limits (VSLs) and ramp metering (RM) have been proved to be effective alternatives to alleviate the traffic congestions occurred on freeways in many studies. However, no research has so far evaluated the efficiency of the above strategies on urban expressways in Chinese cities, where the early onset breakdown often occurs at the bottlenecks because of different driver behaviour from those reported in overseas. Hence, this study was intended to propose an integrated control strategy of VSL and RM to dynamically postpone the early onset breakdown of on-ramp bottlenecks based on its occurring probability. The proposed strategy is capable of simultaneously optimising mainline speed and RM rate based on the modified METANET model and the ALINEA model. Meanwhile, an online simulation platform that integrates VISSIM, Visual Basic and MATLAB programs was developed for the purpose of a close-to-reality evaluation. Empirical results on urban expressways in Shanghai show that the average travel time and the speed difference were significantly reduced by 8.6 and 41.4%, respectively; the pre-queue flow at the bottleneck was increased by 14.7%, which indicates that the early onset breakdown could be efficiently postponed or prevented by the use of the proposed control strategy.