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Transportation Troubleshooting: Coordinated and Adaptive Ramp Metering for Faster, Safer Highways

Paula Hammond on November 30, 2023 - in Articles, Column

I doubt any driver likes being held back by a traffic signal while trying to enter a highway. But from a regional traffic-management perspective, there’s a lot to like about the potential to use ramp metering to improve traffic flow along congested highway corridors.

What’s the CARM?

With vastly more-powerful traffic sensors, computers and software than in the early years of ramp metering, transportation managers can holistically operate highway corridors and surrounding arterials to mitigate congestion, reduce accidents and lower greenhouse gas emissions. This approach is called coordinated and adaptive ramp metering (CARM), and I recently had a chance to get up to speed with WSP experts working on CARM in the United States and Australia.

“Ramp metering systems currently deployed have a wide range of technological sophistication, but a common limitation is that they focus only on the conditions close to the ramp,” explains Christopher Barrow, WSP business leader for Kentucky, who managed a major upgrade to ramp meters in Atlanta. “A centralized adaptive corridor-wide system pulls in data from across an entire corridor and adjusts the meters’ behavior so dozens of ramps work together.”

This means, for example, that when the backup at one ramp becomes severe, the system will adjust upstream ramp meters to temporarily constrain the number of vehicles allowed on the highway to ease downstream congestion and allow more cars at the backed-up ramp to enter.

“Conversely, if a ramp has excess capacity, upstream ramps can be signaled to increase throughput,” adds Chris Swenson, senior vice president and a managed lanes and congestion pricing leader for WSP. “When people think about ramp metering, they usually envision the meters holding vehicles back. But with CARM, it’s as much about getting vehicles on the highway in the right times and places. This maximizes flow on the freeway while minimizing queues on the ramps.”

Permanent deployments of CARM are rare. While California, Minnesota, Maryland and Arizona have versions of adaptive ramp metering in place, the full power of today’s computer, detection and communications capabilities aren’t being used by most agencies. While any ramp metering can be beneficial, allowing the entire corridor to function as a system rather than as individual ramps is key to getting the best solutions.

A Victorian Crown

In most of the United States to date, CARM has been limited to pilot projects and studies to model the benefits of CARM on highway corridors. The most-advanced CARM system that WSP is aware of is in Melbourne, the capital of Victoria and Australia’s fastest-growing city.

Initiated more than 15 years ago by the Victoria Department of Transport and Planning (VicDOT), Melbourne’s “managed motorways” program uses a suite of algorithms to make real-time adjustments to ramp meters based on corridor conditions.

When combined with variable speed limits, lane-use management systems and other intelligent transportation systems (ITS) features, Melbourne’s M1 Motorway saw traffic flow increase by 5-8 percent, speeds by 35-39 percent and travel-time reliability by 150-500 percent during peak periods—while five-year average crash rates declined by 31 percent, according to Lachlan Gray, senior principal ITS for WSP Australia.

A Growing CARM Offensive

Melbourne’s experience has been noted by transportation agencies globally, and one state that was particularly inspired was Colorado. Partnering with WSP, VicDOT and Queensland’s Department of Transport and Main Roads’ ITS affiliate Transmax, the Colorado Department of Transportation created the Smart 25 Pilot Project to test CARM on a 14-mile section of Interstate 25 in metro Denver. After wrapping up in July 2022, an evaluation showed that the nine-month pilot decreased the severity and duration of congestion by 14.3 percent and reduced overall travel times by 19 percent for afternoon peak-traffic periods.

WSP and other transportation consulting and engineering firms are conducting feasibility studies for CARM across the United States. In one notable project, my colleagues deployed our internally developed ITS modeling system to calculate the benefits of CARM for the heavily congested 20-mile Interstate 40 corridor between Raleigh and Durham, N.C. The model predicted 15-18 percent improvements in average speeds during peak congestion periods in many segments. Across the corridor, total travel times would be reduced by 6.25 percent during morning peaks and 7.5 percent during evening peaks.

My colleagues report growing interest in CARM among transportation agencies, especially those with ramp metering systems that aren’t yet coordinated and adaptive. With the data from Melbourne and Colorado showing the benefits that can be achieved—at costs that are a tiny fraction of adding lanes—I’m not surprised to hear this.

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About Paula Hammond

Paula Hammond is senior vice president and national multimodal market leader, WSP USA; email: [email protected].

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