By Alexander Perry, Contributing Author
Most American cities are faced with a congestion problem. A steadily increasing population in urban areas has created more delays on highway corridors.
Part of the dilemma is that traditional ramp metering systems are limited to only specific points of traffic. That’s why more agencies are looking for more holistic approaches to mitigate traffic congestion, as well as reduce accidents and greenhouse gas emissions.
Fortunately, coordinated and adaptive ramp metering systems can deliver results. Christopher Barrow, Kentucky local business leader for WSP, said that these sytems can help to manage the flow of traffic through critical roadway sections.
“A coordinated and adaptive ramp metering system, when executed properly, can help identify critical bottlenecks in transportation systems and allow transportation managers to evaluate the status of those bottlenecks in real time, allowing them to make decisions at entrance ramps that are upstream of them,” Barrow said.
Modern examples of coordinated and adaptive ramp metering build upon previous work in managing roadways with intelligent transportation systems (ITS).
ITS can improve the maximum sustainable flow rate for these projects, including features such as queue warning, dynamic speed limits, advisory signage, and dynamic lane assignment.
The goal is to manage traffic by continuously maximizing the capacity of a motorway network, which requires tracking not only the overall flow of traffic, but also speed and throughput changes at key junctures, like entrance and exit ramps.
One goal of having a coordinated and adaptive system is to have all ramps in a designated corridor metered, so that the detection throughout the corridor is dense and accurate, according to Chris Swenson, WSP senior vice president and southeast director of toll systems and managed lanes in the U.S.
“The system must also update metering rates multiple times per minute,” Swenson said. “And these corridor metering rates must also consider overall corridor traffic conditions and can be adjusted at one (or more) meters, which allows another ramp to release more vehicles to prevent excessive backup on any ramp.”
Most ramp metering systems are limited to just the conditions around the ramps themselves. But a coordinated and adaptive ramp metering system is centralized and corridor-wide, allowing it to be more adaptive by pulling in data across an entire corridor to adjust the meters’ behavior.
“The biggest difference is how much information is being brought in across the corridor to make a better decision,” Barrow said. “The key is controlling demand during peaks.”
The approach requires continuous real-time data on roadway performance across all lanes and in specific locations along the corridor. This feeds the centralized control algorithm that then decides the best way to manage the system via coordinated ramp metering, variable speed limits and lane-use management systems.
When there’s a sizable back-up at one ramp, the system can adjust upstream ramp meters to temporarily restrict vehicle access to the highway. This in turn eases downstream traffic congestion and allows more cars access at the backed-up ramp.
“While a traditional ramp metering system just tries to make one individual access spot as good as it can be, coordinated and adaptive metering tries to make the entire corridor as good as it can be,” Swenson said. “People often think of ramp metering as simply holding traffic off a roadway, and while there’s certainly some of that, it’s also equally about moving traffic onto roadways as efficiently as possible.”
Coordinated and adaptive ramp metering is limited to pilot projects and studies across the U.S., with one notable case study being the Smart 25 Pilot Project in Denver.
The Colorado Department of Transportation (CDOT) implemented this pilot on a 14-mile section of Interstate 25 (I-25) in metro Denver. This was based on the concepts that were originally created in Melbourne, Australia, where the Victoria Department of Transport and Planning (VicDOT) founded one of the world’s most advanced coordinated and adaptive ramp metering systems.
Initiated 15 years ago, VicDOT’s managed motorways program yielded impressive results for Melbourne’s M1 Motorway. Traffic flow on the M1 increased by 5-8%, speeds by 35-39% and travel time reliability by 150-500% during peak periods. Furthermore, five-year average crash rates dropped by 31%.
For Smart 25, CDOT partnered with WSP, VicDOT, Queensland’s Department of Transport and Main Roads’ ITS affiliate Transmax.
For the pilot project, CDOT installed 37 high-definition data detection systems on this 14-mile stretch, and it collected the data necessary for smarter corridor management by the ITS, according to Barrow. There are now 14 ramps metered in the corridor, which are property of CDOT.
The nine-month pilot project officially concluded in July 2022 with a positive evaluation: the severity and duration of congestion on the I-25 stretch had decreased by 14.3%, and the overall travel times had dropped by 19% during afternoon peak traffic times.
Transportation consulting and engineering firms are conducting feasibility studies for coordinating and adaptive ramp metering across the U.S. Other states (including Georgia, North Carolina, California, Utah, and Kentucky) have considered piloting similar concepts.
Another notable project focuses on the heavily congested Interstate 40 corridor between Raleigh and Durham. It is past feasibility and moving along towards identifying construction and implementation funding.
For the planning phase of this project, the North Carolina Department of Transportation used a proprietary corridor modeling system to identify where it should invest its resources based on some critical policy decisions.
The model does not require large amounts of data, and it allows ramps to interact in the model in the same way they do during actual operation.
Constraints such as available storage and maximum allowable wait times can be input. Multiple model runs are automatically made simulating how arrivals at the on-ramps will vary. This allows an agency to identify where ramp or mainline improvements, or additional strategies, such as hard shoulder running, will be beneficial.
In California, the Riverside County Transportation Commission (RCTC) is set to launch an $18 million smart freeway pilot project on Interstate 15. This is expected to be the second coordinated and adaptive ramp metering project nationwide, following Colorado’s Smart 25.
The project covers the congested corridor between the San Diego County line and through Temecula in the northbound direction.
RCTC has been seeking improvements to address this congestion, including participation in Temecula’s French Valley Parkway interchange improvement project that overlaps with this pilot.
“Due to the lane geometrics, combined with the heavy congestion, there’s bottlenecking occurring in the corridor,” said Melissa Brady, vice president and civil engineer for WSP. “Construction is anticipated to start this summer for the implementation of a coordinated and adaptive metering system, including three specific interchanges, in order to improve the operational flow along this nine-mile section of the corridor.”
Coordinated and adaptive metering has gained interest among transportation agencies. However, there are notable challenges that need to be considered for implementation.
The Melbourne approach, for example, requires a range of physical infrastructure to create space for vehicles on ramps and possible lane allocation around ramps. This is in addition to ITS devices and control systems that support pre-emptive control beyond the motorway.
The approach also requires a keen understanding of traffic flow, behavioral science and continual study and optimization. Lastly, a robust operational framework is required to ensure the results can be delivered reliably for road operators.
Barrow said that part of the challenge is identifying the best equipment that works in a particular environment, so that roadway owners have the necessary vehicle detection data to support a smart system to manage these freeways.
Despite the implementation challenges, coordinated and adaptive ramp metering has the potential to deliver significant benefits to congested freeway corridors across America.
“The approach can reduce travel time delay, improve travel reliability and safety for people and critical goods, and reduce emissions with fewer stop-start traffic patterns on these highways,” Brady said. RB
