David Levinson, Professor, Civil, Environmental and Geo-Engineering
Traffic congestion has become an increasingly serious problem in many cities. Ramp metering, which maintains smooth freeway mainline flow by limiting vehicle entry at entrance ramps, has been proposed and implemented in a number of metropolitan areas in and outside the U.S. to mitigate freeway congestion. This study aimed to develop both efficient and equitable freeway ramp control strategies. Traffic conditions with and without ramp metering were evaluated on several representative freeways in the Twin Cities with a comprehensive set of performance measures. A unified theory for ramp metering was then proposed, based on a linear programming model of freeway traffic dynamics. The most efficient ramp control algorithm was found to be also the least equitable one. A novel control objective, minimizing weighted or perceived travel time, is therefore proposed to balance efficiency and equity objectives of ramp metering. This research also developed a new family of applicable ramp metering strategies, which consider both efficiency and equity, and demonstrated these strategies in a microscopic traffic simulator. Future studies should compare various traffic control methods under the analytical framework proposed in this report. Researchers should also pursue field experiments of the proposed multi-objective ramp control strategies.