What is Ramp Metering?

A ramp meter is a traffic signal located on a motorway on-ramp that regulates the traffic entering the motorway from that ramp. The objective of ramp metering is to reduce congestion on the motorway and to allow traffic joining the motorway to merge more easily and more safely.

Click here for a short movie file of ramp metering in operation in the Netherlands. The ramp meter shown in the video (at the S101 on-ramp to the A10-West near the Coentunnel) has been in operation since 1989. The start of the video shows merging behaviour before ramp metering and this is followed by video of merging behaviour after ramp metering was implemented.

Depending on the layout of the on-ramp there can be one or two lanes of queuing traffic. In general, when the signal turns green one car in each lane is allowed to go. However, some ramp metering locations operate a "two cars per green" policy.

In a fixed-time ramp metering strategy, the traffic signal settings for the metered ramp are calculated offline based on previously-collected data on traffic demands at that location. There will be different signal settings for different times of day and for different types of day (weekday, weekend, holiday).

In a reactive ramp metering strategy, the signal settings are calculated based on real-time traffic data from detectors on the ramp and on the highway upstream and downstream of the ramp. According to the precise strategy adopted, this detector data is processed to produce the signal timings at the ramp, which are therefore reactive to changing traffic conditions. Predictive ramp metering strategies go one step further by using real-time traffic measurements along with appropriate estimation and prediction algorithms. Predictive strategies therefore take control actions in real-time to deal with anticipated problems.

Ramp metering can either be applied in isolation to an individual on-ramp (local strategy) or a co-ordinated strategy could apply to several ramps.

page maintained by Helen Condie at TRi, Napier University, last updated 1/11/06

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