To reduce expressway traffic congestion caused by traffic demands in excess of capacity, an experimental system of four entrance ramp metering devices was installed on Chicago's outbound Eisenhower expressway. Entrance ramp traffic is automatically metered each peak period by modified traffic signals which permit only one vehicle at a time to enter the expressway-ramp merging area. Metering rates are determined through computer analysis of existing expressway traffic conditions as measured by a surveillance detection system. Favourable expressway operational benefits attributable to the ramp control system included reduced traffic congestion, decreased travel times, increased speed, increased upstream flows, maintenance of bottleneck throughputs and a trend towards decreased accidents. While providing a compliance rate above 95 percent, the ramp signals produced some adverse effects by delaying and diverting ramp traffic. Ramp queuing increased delays on the metered ramps and at the ramp-arterial intersections, but the overall contiguous surface street network was not noticeably influenced by diverted traffic. The experimental traffic control system demonstrates that automatic entrance ramp metering can be a practical remedy for congestion caused by expressway overloading. Care must be taken with applications, however, as the degree of ramp control required must not produce effects negating the expressway benefits derived. /author/.
Abstract