Traffic congestion occurs when travel demand exceeds capacity. Such congestion is a daily occurrence on much of the urban freeway network. To alleviate traffic congestion due to recurrent congestion, ramp metering has been applied in numerous cities in the United States as well as throughout the world. Ramp metering aims to limit the number of vehicles entering the freeway from entrance ramps at specific periods of time so that freeway flow can be maintained at or better than a desired service level.
Excess demand is forced to wait at the entrance ramp.
The intention of ramp metering is, therefore, to maintain uninterrupted, non-congested flow on the freeway as long as possible by transferring delay from the freeway to the entrance ramp . As a result of ramp metering some traffic will divert to alternative routes or perhaps to alternative departure times. It is highly unlikely that any modal diversion would occur. Various positive effects of ramp metering systems on freeway traffic have been reported based on comparisons of traffic flow operations before and after ramp metering. However, those positive effects reflect only traffic improvements caused by ramp metering systems, and fail to include the costs imposed by ramp metering.