Probability of Capacity Enhancement and Disruption for Freeway Ramp Controls

Abstract

Models based on gap-acceptance and queuing theory are proposed to model the effect of ramp controls on freeway operations. The models were developed for three types of patterns in ramp control and traffic flow: the uniform arrival with ramp metering, the random arrival without ramp metering, and the platoon arrival without ramp metering but with an upstream signalized intersection. From the theoretical point of view, the models were applied to address freeway ramp metering thresholds and the corresponding disruption probability. Studies have shown that ramp metering has been effective in reducing vehicle delays only when the rate of freeway traffic flow has reached a certain level of flow threshold. When freeway traffic is low, there are enough gaps in the freeway flow to accommodate the ramp flow, even when ramp traffic enters the freeway in platoons. The presented models accounted for the effect of platoon size that resulted from the three ramp controls and arrival flow patterns. Study results clearly indicate that more significant disruption on freeway operations occurs as a result of large platoon arrivals caused by an upstream traffic signal, than as a result of traffic that arrives randomly or uniformly. The models were also applied to provide quantitative assessments from the perspectives of freeway capacity. The results indicate that ramp metering produces increased freeway capacity and decreased disruption probability.