Are Minimization of Delay and Minimization of Freeway Congestion Compatible Ramp Metering Objectives?

Analysis of mechanisms by which ramp metering can reduce traffic delay, such as increasing bottleneck flow, expediting flow to exits upstream of the bottleneck, and diverting traffic around the bottleneck, suggests that minimization of delay and minimization of congestion on the freeway main line are not always compatible objectives. Simulations were performed to tentatively resolve this question. These simulations used a flow model based on Newell’s simplified kinematic wave theory and a control strategy that was a modification of that of Wattleworth and Berry. Simulation scenarios defined physical characteristics, demand patterns, and flow characteristics for hypothetical freeway sections on the basis of data from Blumentritt et al. and the San Diego area. Results of the simulations show that there are circumstances under which minimization of delay does not coincide with minimization of congestion on the main line and that relationships between delay and main-line congestion depend on the combination of the metering mechanisms in effect.