Metering Ramps to Divert Traffic Around Bottlenecks: Some Elementary Theory

Three elementary cases, with ramp metering used to reduce delay by diverting traffic around bottlenecks, are analyzed. In these cases (a) travel times on an alternate route bypassing the bottleneck are insensitive to flow on the alternate route, (b) the alternate route is undersaturated but travel times are sensitive to flow, and (c) the alternate route is oversaturated. Travel time equilibria and traffic assignments are relatively straightforward in all cases provided that equilibria in Cases b and c are assumed to be approximate and traffic assignments are based on drivers’ expectations about traffic conditions prevailing at particular times of day. A metering strategy intended to minimize delay is proposed. This strategy is expressed in terms of the order in which metering is initiated at different ramps and is similar to one previously proposed to maximize output to exits upstream of the bottleneck.