Ramp Metering and the Capacity of Active Freeway Bottlenecks

The objective of this study is to determine whether ramp meters increase the capacity of active freeway bottlenecks, and if they do, how. The traffic flow characteristics at twenty-seven active bottlenecks in the Twin Cities have been studied for seven weeks without ramp metering and seven weeks with ramp metering. A series of hypotheses regarding the relationships between ramp metering and the capacity of active bottlenecks are developed and tested against empirical traffic data. It is found that meters increase the bottleneck capacity by postponing and sometimes eliminating bottleneck activations (a 73% increase in the duration of the pre-queue transition period), accommodating higher (2%) flows during the pre-queue transition period, and increasing queue discharge flow rates after breakdown (3%). The two-capacity hypothesis about flow drops after breakdown was also examined and results strongly suggest the percentage flow drops at various bottlenecks follow a normal distribution (mean 5.5%, standard deviation 2.3%). The implications of these findings on the design of efficient ramp control strategies are discussed, as well as future research directions.