TWO-CAPACITY PHENOMENON AT FREEWAY BOTTLENECKS: A BASIS FOR RAMP METERING?

The issue of whether ramp metering can increase the capacity of freeway bottlenecks by prevention or delay of flow breakdown on the freeway main line is considered. A summary of the results of four case studies of metered bottlenecks in San Diego is presented. The hypothesis that flow decreases when it breaks down is confirmed, provided the hypothesis applies to individual lanes. Flow decreases ranging from 10% to less than 1% were observed in the left lane at the various study sites. When averaged across all lanes, flow decreased by about 3% at one site; there was no significant change at the other sites. It is concluded that this phenomenon is unlikely to provide a basis for metering at more than a few locations, because decreases in flow across all lanes are very small, sometimes nonexistent. Even where there are decreases in total flow, there is risk that metering will be counterproductive if it is too restrictive or begins too early.