Ramp Metering: A Proven, Cost-Effective Operational Strategy—A Primer

2. Ramp Metering Challenges

Though ramp metering has proved a successful strategy in many cities, agencies continue to face barriers and other opposition in their attempts to deploy or expand their ramp metering systems. In a survey of the top metro areas, the barriers cited by agencies displayed common themes, such as lack of support—whether financial or political—and existing infrastructure that was not compatible with ramp metering.

Figure 7 shows the most common barriers to development from the survey.

Figure 7: Identified barriers to ramp metering deployment

Source: Parsons Brinckerhoff.

Typically, regions with existing ramp metering systems will see areas of opportunity where ramp metering could be further deployed to improve freeway operations.

Of the regions that already have ramp meters in operation, many cited barriers to expanding their current system. The most prevalent barrier was the geometric limitations of the existing ramps (58 percent), with capital costs (42 percent) and operations and maintenance costs (26 percent) also frequently cited. The ability to maintain the system infrastructure and the technical skills of the existing staff were seen as barriers in 16 percent of the regions. Public opposition and local agency opposition was cited by 11 percent of regions as barriers to expansion.

2.1 Geometry of Existing Infrastructure

Some agencies have stated that ramp metering is not possible in their region due to the configuration and structure of their ramps. Because ramp metering requires space for vehicles to merge into mainline traffic and to wait in a queue, not all ramp configurations are suitable for ramp metering. Key geometric issues that should be considered when investigating ramp metering include inadequate acceleration length, mainline weaving problems due to closely spaced ramps, and limited sight distances on a horizontal or crest vertical curve. Ramp metering can reduce the impact of geometric conditions that cause problems under high speed or high volume ramp conditions and heavy mainline volumes because the ramp speeds are likely to be lower and there is less disruption from vehicle platoons vying for limited mainline gaps.

Figure 8: Example of challenging geometrics for ramp metering

Source: Parsons Brinckerhoff.

For instance, if an on ramp lane merges too abruptly, the short distance past the stop bar may create unsafe conditions leaving inadequate space for vehicles to accelerate to a safe speed in comparison to the mainline. Figure 8 illustrates common geometrics that pose challenges to ramp meter implementation. In addition, narrow configurations may not have adequate shoulder space for an HOV bypass lane, a transit bypass lane, or an enforcement zone where compliance can be monitored.

2.2 Costs and Funding

Despite the benefits of ramp metering, there are monetary costs for deploying and maintaining ramp metering systems. Without sufficient and well-timed funding, ramp metering deployment may be unsuccessful and not realize its full benefits. When agencies attempt to procure funding for ramp metering programs from local, state, or federal governments, they are often competing with other initiatives that may have greater priority or support. As exhibited in recent survey results, most agencies desire expansion of their ramp metering system following the initial deployment, which would require additional funds.

2.3 Public Opposition

The support of the public is often critical in the deployment and expansion of ramp metering. Many agencies encountered public opposition that significantly deterred or completely halted efforts to deploy ramp meters. Often this opposition is rooted in misconceptions about ramp metering and its effectiveness. The public may also raise concerns about equity, claiming that ramp metering asymmetrically favors those who live outside the city center.

2.4 Heavy Ramp Volume

As a ramp meter regulates the flow of vehicles entering the mainline, cars form a queue on the ramp behind the signal. If the meter’s release rate is less than the rate at which vehicles approach the ramp, the queue will lengthen. If too long, a queue could spill onto arterials resulting in inefficient arterial operations. Ramps that are shorter in length or have less storage space are at a higher risk of arterial backup than long ramps with similar demand.

2.5 Local Agency Opposition

Deploying or expanding ramp metering systems often impacts a variety of stakeholders including arterial agencies and local businesses. These groups may have a negative perception of ramp metering and may have the resources to halt efforts to deploy or expand ramp metering. Local agencies are often concerned that the ramp queues will negatively affect their transportation systems. Local agencies, like the public, may also be concerned about equity issues. Conversely, such groups would be valuable allies in building support for ramp metering and ensuring smooth operations.

2.6 Lack of Agency Support

Sometimes there may be a lack of support from the agency with responsibility for freeway operations. There could be many reasons for this lack of support. Sometimes it is because key people in the agency do not understand ramp metering. They may not be aware of the benefits of ramp metering and the high benefit/cost ratio that ramp metering projects show. It may also be because there is a concern over long term maintenance and operations costs. Agency staffing may also be an issue, either there is not sufficient staff or there is not staff with the needs skills and knowledge to implement or operate ramp metering. Consistent and clear communication and information can often increase agency support.