Ramp metering is an active traffic management (ATM) strategy that uses traffic signals at freeway on-ramps to control the number of vehicles entering the freeway to keep vehicles moving more efficiently.
Key characteristics
Other names
- Ramp flow control
Strategy description
Ramp metering uses specialized traffic signals located at freeway on-ramps that alert drivers to stop on the ramp before merging onto the freeway. By limiting (or “metering”) how many vehicles can enter the freeway at a given time and by organizing the entering vehicles in a more regular and even pattern, ramp metering helps traffic merge onto the freeway more smoothly and efficiently. Regular and even gaps between vehicles promotes a zipper-like merging pattern, where ramp and freeway vehicles alternate one-by-one for position throughout the merge, which minimizes stop-and-go traffic at the ramp merge point.
Ramp meters typically operate by allowing one car to proceed per green light. They help control the flow of traffic onto the freeway by varying the amount of time between green signals. The longer the wait between green lights, the fewer number of cars can enter the freeway per minute. The metering rate chosen must be balanced against the risk of backing up traffic on the ramp and into the adjacent streets.
Ramp meters can be active at all hours, but are typically used only during peak traffic periods when they have the greatest potential to improve freeway congestion conditions.
The most straightforward ramp meter systems operate on a fixed metering rate, providing a pre-set interval between green signals. More advanced, dynamic systems use real-time freeway and adjacent arterial detection to automatically adjust and optimize metering rates based on real-time congestion and capacity observations.
When to use this strategy
Ramp metering makes sense for:
- Freeway corridors with regular, recurring congestion
- Stop-and-go traffic conditions that occur downstream of a freeway entrance ramp
- For ramps that have large volumes of traffic that arrive at a given time, resulting in bottlenecks at the freeway entrance
- Ramps that are long enough to operate safely and efficiently, including space to accommodate a queue of vehicles, as well as the necessary distance after the signal to accelerate up to a safe merge speed
- Ramps where the number of vehicles is not so high that ramp meter queues would regularly back up to the nearby intersection and adjacent streets
While effective ramp metering does not eliminate traffic congestion, it does delay the onset of congestion, as well as shorten the duration (i.e., shrinks the rush hour window). Better organized zipper merging and less stop-and-go traffic can mean fewer collisions, making freeways safer.
Strategy benefits:
- Delays the onset and shortens the duration of recurring freeway congestion
- Increases safety by reducing the likelihood of stop-and-go traffic collisions
- Increased freeway volume and speed that reduces travel times for all vehicles
- Relatively low cost to install and maintain
- Reduce emissions and fuel consumption by reducing stop-and-go traffic
What you need in order to implement
Planning needs:
- Operational analysis to consider the demand, congestion, and safety conditions and to determine appropriate metering strategies
- Design analysis to confirm sufficient acceleration distance to merge onto the freeway, ramp length for accommodating the queued vehicles, and stopping distance from the nearby signal to the back of the ramp queue
Coordination needs:
- Coordinate with the local traffic agency operating the streets connected to the freeway ramp in order to agree on operating rules of the ramp meter and how to deal with impacts to adjacent street traffic
- Coordinate with law enforcement to address potential violations
- Media and public outreach to provide education if the concept is new to drivers in the area
Equipment needs:
- Ramp meter signals
- Ramp metering software
- Vehicle detection on the ramp and freeway lanes to measure traffic volumes
- CCTV cameras for verification and monitoring
Agency resources needs:
- Agency staff resources to install, calibrate, and operate ramp metering equipment
Learn more about this strategy
Federal Highway Administration (FHWA), Ramp Management website.
Federal Highway Administration (FHWA), Ramp Metering: A Proven Effective Strategy pamphlet, 2014 (PDF).
Federal Highway Administration (FHWA), Ramp Metering: A Proven, Cost-Effective Operational Strategy primer, 2014 (PDF).
About key characteristics
Location notes:
Ramp meters are appropriate for freeways access points.
Cost notes:
Ramp meter costs can vary based on the sophistication of the system, but are generally low in relation to other technology and infrastructure strategies.
Technology notes:
Technology needs can vary depending on the sophistication of the ramp metering system, but typically require local traffic detection, a computing component for running ramp metering software, and communications to the local ramp meter signal, which make ramp meters a mid-range cost solution.
Collaboration notes:
Ramp metering systems typically require some collaboration with the local traffic agency in order to determine how best to operate the system without negatively impacting local streets.