PATH ITS Research Digests

Ramp meters are commonly used to relieve congestion on freeways. What is the value of these systems? Ramp metering can alleviate recurrent, regular traffic congestion, such as that caused by the daily commute. The goal is usually to maintain uninterrupted freeway flow at a certain acceptable level, but it transfers delay to the entrance ramp, where excess demand is forced to wait. This report summarizes ramp metering technologies, reviews the algorithms commonly used with them, and uses a traffic simulation model to identify the benefits and costs of using these systems. We used a combination of experience from 10 different sites using ramp metering and a traffic simulation model to identify and test costs and benefits. Because our assumptions produced positive results, we changed some of them to reduce benefits, but even so we found ramp metering to be worth implementing. The changed assumptions included reduced ramp demand, reduced freeway demand, and reduced capacity. Benefits from using ramp metering systems include increased speed of traffic, travel time reductions, accident reductions (as high as 50%), and an increase in traffic flow, which by reducing stop-and-go conditions on freeways also reduces fuel consumption and some emissions—even when the increase in fuel consumption caused by vehicles waiting on the ramp is accounted for. However, ramp metering also causes a general increase in emissions.