What Is It?

• Ramp metering is the use of traffic signals at freeway on-ramps to control the rate of vehicles entering the freeway.

• The metering rate is set to optimize freeway flow and minimize congestion. The metering rate can be fixed, or responsive to local or system-wide conditions.

• Signal timing algorithms and real-time data from mainline loop detectors are often used for more effective results. See our Telecommunications Diagram on Ramp Meters for more information.

Key Results

In practice, ramp metering systems have been extremely successful in reducing congestion and increasing safety. Most result in higher mainline throughput with lower congestion, significant travel time savings, and higher travel time reliability. However, effects on fuel consumption and emissions have been mixed. The reduced congestion on the freeway allows for greater fuel efficiency and reduced emissions once on the mainline, but vehicles queued at ramp meters have increased rates of fuel consumption and emissions.Ramp metering algorithms have some limitations, which researchers are working to eliminate. One problem is that existing algorithms react to rather than prevent bottlenecks. This causes oscillatory behavior, as a result of the time lag between detection and corrective action. A proposed solution involves integrating traffic predictive capabilities into the metering logic. Several such algorithms employ neural networks and Fuzzy Logic techniques, and can potentially delay or prevent bottleneck formation. 

Benefits

• Travel time savings
• Improved safety
• Effective use of capacity
• Reduced travel time variability

Costs

Costs to the implementing agency include the development and calibration of metering system algorithms as well as infrastructure and maintenance expenses. In addition, a control center may be desired to monitor the system as a whole. The cost of a particular ramp metering system varies widely according to the sophistication of the algorithm used to set the metering rate and the number of ramps included in the system. Potential costs to the public are:

• Undesirable levels of traffic diversion to surface streets
• Increased emissions and/or fuel consumption at ramps
• Ramp delay and spillback onto surface streets
• Promotion of longer trips
• Transfer of land values

Implementation Challenges

Implementation of ramp metering is almost always initially opposed by the public because of increased queues at on-ramps. The most successful programs have instituted large campaigns to educate the public on the benefits of ramp metering. In addition to this initial opposition, equity issues may arise due to the fact that ramp metering often promotes longer trips over shorter ones. This problem may be addressed by adjusting metering rates. Finally, existing ramps must have enough capacity to accomodate increased ramp queues without causing excessive spillover onto the surface street network. If ramps do not already have this capacity, construction costs can be formidable.

Where is it implemented?

Ramp metering is implemented across the United States and Europe.

Author: Rebecca Pearson