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Researchers Conduct Original Work-Zone Ramp Metering Study

by Kyle Harpster

missouri

If you’ve driven in California, the Twin Cities, or any of several other metropolitan areas across the country, then you may be familiar with ramp meters. Ramp meters are signal lights on entrance ramps designed to improve the efficiency of traffic merging onto a freeway or highway. Studies have shown that they are beneficial because they can shorten travel times and decrease delays.

Until recently, however, there have been no studies on the use of ramp metering in work zones in the United States. MATC researchers from the University of Missouri-Columbia, including Associate Professor of Engineering Carlos Sun, Assistant Professor of Civil and Environmental Engineering Praveen Edara, and engineering graduate research assistant Zhongyuan Zhu, recently evaluated the use of temporary ramp meters near work zones in order to learn more about their safety and efficiency.

In 2011, the researchers deployed temporary ramp meters at seven short-term work zones near five different ramps on I-70 and US 63 in Columbia, Missouri. The team set up four video cameras and two speed radars at each location to gather data at the merging area and the ramp during under-capacity, at-capacity, and over-capacity traffic flow conditions. The researchers collected data on several surrogate measures of safety, including driver compliance rate, speed at the mainline and ramp, and speed differentials between merging vehicles and mainline vehicles. The results showed that about 34% of drivers failed to comply with the ramp meters, which could present a significant safety problem. The team also noted a “platoon compliance effect” where if the leading vehicle complied with the meter, all of the following vehicles also complied. The researchers also discovered that a three-section signal head resulted in higher compliance rates than a two-section head.

“Since this was the first study of temporary ramp metering at work zones, we did not know quite what to expect,” Sun said. “So we were not necessarily surprised by the compliance rate during free flow conditions. Also, since Missouri uses very few permanent ramp meters, it is not surprising that drivers seem to understand the three-head better than the two-head signal.”

The researchers say the results of this study will help inform state transportation agencies on the potential advantages and disadvantages of using temporary ramp metering in work zones. They concluded that temporary ramp meters should only be deployed in places where congestion could occur, and should only be turned on during above-capacity conditions. For at-capacity or under-capacity conditions, the ramps increased delays. But during heavy traffic, the ramp meters decreased delays for both mainline and ramp vehicles, and prevented platoons from merging onto the mainline.

“I think it was important to determine the parameters under which this strategy could function,” Sun said. “So using a three-section head with the prescribed signal timing during congested periods seems to be the optimum use in Missouri. It would be counterproductive to have the meter on when the mainline and ramp volumes do not justify its use.”

Notably, the ramp meter deployments in this study were all conducted during off-peak hours because the Missouri Department of Transportation avoids closing lanes in urban areas during peak hours. The researchers suggested that the current study could be supplemented by a future project examining the use of ramp metering at congested field sites.

“At this point, there is nothing planned for a follow-up study,” Sun said, “although we would be interested in conducting such a study if a congested site became available.”

You can click here to view the full report on this research, "Evaluation of Temporary Ramp Metering for Work Zones."