Freeway on-ramp metering, delay savings, and diverge bottleneck

An effort was made to clarify certain issues concerning freeway on-ramp metering and its potential for saving commuter delay. Simple analogies were used to show that delay is reduced if ramp metering increases the rates at which commuters exit the freeway. Contrary to what is frequently reported in the literature, higher travel speeds and flows on links within a freeway system are not evidence of diminished delay and this too was made clear by the analogies. The discussion explains why a metering scheme should be specially tailored to the freeway it serves, and why no single metering logic will suffice for all situations. This idea was emphasized by demonstrating that some of the best-known on-ramp metering algorithms can actually reduce freeway outflows (and therefore increase delay) when the freeway is plagued by a diverge bottleneck with a congested off-ramp. Other more effective strategies for dealing with the (commonly occurring) diverge bottleneck are described, as are methods of field-testing these strategies. Changes in commuter travel behavior brought by metering, and possible effects of these changes, are discussed as well.