Impact Of Variable Speed Limit Control On Traffic Operation In Freeway Off-Ramp Function Area

The off-ramp is considered as one of the most important bottlenecks on freeway. When the traffic demand is high, the phenomenon of capacity drop appears, which apparently decrease the traffic flow on the function area of off-ramp. The variable speed limit (VSL) control as a real-time dramatic control system is able to relieve the capacity drop phenomenon. The insight of the relationship between the VSL control and the macroscopic and microscopic traffic flow characteristics is still underdeveloped. The simulation of the off-ramp function area under VSL control is still inaccurate. The comparison between different strategies in SPECIALIST algorithm is still sorely lacking.Those shortages lead to the following aims:First, the impact is analysed on macroscopic and microscopic traffic flow characteristics in off-ramp function area by VSL control including the compliance behaviors of drivers under SPECIALIST VSL control algorithm. Then a cellular automata model is set up to simulate the traffic behaviors under VSL control in off-ramp function area. The Latin Hypercube Design method and the analysis of variance method are used in key parameters selection, and the Genetic Algorithm is used in model calibration. The distribution of speed and time headway are used to validate the model. Loading different VSL control parameters set in the model, the control projects are compared and the control strategy is optimized.The conclusion includes that, the lower VSL value, the higher the critical density is, while the too low value may lead to traffic chaos. As for compliance behaviors, the compliance rate apparently decreases when the VSL value reaches to 80km/h, and the average expected speed overcomes the VSL value when it reaches to 70km/h. Basing on the simulation results, when the density is 5veh/km, the best VSL project includes the speed threshold of 60km/h, flow threshold of 1656veh/h, VSL control distance of 500m, and VSL control duration of 64s; when the density is lOveh/km, the best VSL project includes the speed threshold of 60km/h, flow threshold of 1656veh/h, VSL control distance of 1500m, and VSL control duration of 79s; when the density is 15 veh/km, the best VSL project includes the speed threshold of 50km/h, flow threshold of 1490veh/h, VSL control distance of 500m, and VSL control duration of 72s; under the same density, the best VSL project can also include the speed threshold of 80km/h, flow threshold of 2200veh/h, VSL control distance of 2000m, and VSL control duration of 80s; when the density reaches to 20veh/km, the SPECIALIST algorithm is not valid. This paper takes the off-ramp in Shuangqiaomen interchange area as a case. It turns out that the SPECIALIST control algorithm could remove the capacity drop at bottleneck area, and improve the traffic stability at VSL control area.