Ramp Meters

Case Materials by Norah Montes de Oca, Jeremy Tenant, Jeff Kurth

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Ramp meters are used to control the access to freeways during peak hours. By restricting access, ramp meters increase traffic flow on the mainline. As a result, many cars are left waiting on the ramps.

Many debates arise form ramp meters. The first is simply do they work? Are ramp meters actually reliving traffic tension, or do they just change the location of tension? Many argue that meters cause more traffic in local streets. Also, for some travelers, much time is spent waiting of the ramps. Another debate is whether metering is fair. It may take some individuals longer to reach their destination, while others are able to travel for shorter periods of time.

One misconception about ramp meters is that they should prevent congestion on the mainline. Actually, to optimize total travel time, mainline flow must experience some delays. This will keep the wait time on the ramps down. The problem really lies in reaching the optimization. Unfortunately, minimizing total travel time does not mean minimizing everyone's travel time. Some individuals will experience more delays from metering (Zhang).

The complexity of the current freeway system makes it hard to find optimal ramp metering answers. The theoretical goal for freeways would be to reach capacity, but there are many problems associated with this. One problem with reaching capacity is that the users experience a decrease in the quality of service. The capacity of a freeway is not at free flow speed. Traveling at less than free flow speed is not favorable for most users. Since freeway capacity will require users to spend more time driving, a balance between freeway efficiency and freeway speeds need to be reached. It might be cheaper for more freeways to be built, if it means less time wasted in traffic (Zhang).

There are many alternatives to ramp metering. One is the do nothing approach. Another would be to add lanes and freeways to the system. Or mass transit could be upgraded and encouraged. Whatever the solution, the problem of congestion on the freeways is real, and needs to be addressed.

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