One of the themes in Traffic is the difficulty individual drivers can have in understanding how the system as a whole functions. Ramp meters are a perfect example: Many drivers, particularly at the moment they are asked to pause at the ramp-meter light before joining the freeway, are under the impression that they make congestion worse than if the highway were left to its “natural” state.

But a recent example from Atlanta provides yet another example of how ramp meters generally help, not hurt, traffic flow. Drivers may have to wait briefly on a ramp, but this typically means a faster trip on the “main line.” The concept is in some ways similar to way “congestion windows” are used to manage things like Internet traffic, holding up bits of incoming data if the network is already crowded.

Atlanta, by way of introduction, home of some of the U.S. worse congestion, has embarked on a ramp-meter spree, with 70 new signals coming online. In a recent paper presented at the Institute for Transportation Engineers conference (by Marion G. Waters III at Gresham, Smith and Partners), I came across this curious example of “before and after” (the after is in the photo above):

“A completely unexpected event occurred in March 2008 to validate the benefits of the existing ramp meters in use in Atlanta and to encourage their use in a system of traffic management.

A tornado hit downtown Atlanta for the first time (or at least in a very long time). It damaged one ramp meter and virtually destroyed another one. This was two of the four ramp meters being operated as a traffic-adjusted system on southbound I-75/85 in the downtown area.

The results were dramatic. Congestion on the main line of the freeway was noticeably worse, and the ramp congestion at the adjacent ramps became worse because the main line was completely full. Mainline operating speeds dropped (peak hour operating speeds) and were measured to be lower by as much as 16 mph in the most congested hour.

When these ramp meters were restored to full actuated operation, the conditions were reversed, demonstrating the effectiveness and value of the ramp meters working together in a system along a segment of freeway.”

The report goes on to note that since the meters were turned on in June, “the first indications are that free flow was extended an additional 10–20 minutes.”

This entry was posted on Friday, September 26th, 2008 at 12:54 pm and is filed under Cars, Drivers, Traffic Engineering, Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.