A Ramp Metering Method for Incident Condition using Shock Wave Theory

Ramp meters are traffic signals at the end of freeway on-ramps that regulate the flow of traffic onto the mainline. The primary purpose of ramp meters is to prevent the freeway from becoming over-congested, and to maximize the efficient use of freeway capacity. While the original ramp metering controllers used pre-timed ramp meters, the majority of modern ramp metering algorithms are traffic responsive. Although many studies have been conducted in recent years, most of them focus on bottleneck congestion. Since incidents account for approximately 25% of highway congestion, a study on ramp metering algorithms for incident conditions is necessary. Given the fact of using historical data and their recursive logic, most existing ramp metering algorithms are slow to alleviate unexpected incident congestion.;Based on the relationship between incident queue development and incident shock waves, two shock wave based incident ramp metering methods were developed in this study. The first method reacts to queue end arrivals; the second method proactively decreases incident queue propagation speed by creating cancelling shock waves in the opposite direction. A case study was conducted on a freeway corridor near Milwaukee, WI, using VISSIM simulation software by comparing the proposed methods with three prevailing ramp metering algorithms. The effects of several variables were also investigated including: demand level, incident severity, incident duration, incident location, and loop detector update frequency.