Stop or Go?
Ramp metering has demonstrated that it reduces freeway congestion and crashes. So why aren’t more transportation agencies deploying it?
Despite clear benefits in safety and efficacy, transportation agencies are underutilizing ramp metering as a safety and traffic management strategy. Ramp metering uses signals to control the rate at which vehicles enter a freeway. The strategy received a high benefit rating in the national 2010 Intelligent Transportation System Deployment Tracking Survey, and a number of the survey’s respondents indicated they had plans to implement ramp metering in the future. Yet, of the top 52 U.S. metropolitan areas by population, more than half have no ramp metering in place.
Without ramp meters in operation, multiple vehicles often merge into the mainline in tightly packed platoons, causing drivers on the main highway to slow down or even stop to allow vehicles to enter. The cascading slower speeds, both on the mainline and on the ramp, quickly lead to congestion and sometimes stop-and-go conditions. Ramp meters help to break up the platoons by controlling the rate at which vehicles enter the mainline. This metering enables vehicles to merge smoothly and reduces the need for those on the mainline to slow down.
In addition to breaking up platoons, ramp meters help manage entrance demand at a level that is near the capacity of the freeway, which helps prevent flow breakdowns. Ramp meters are shown to reduce peak-hour lane occupancies (freeway density) and quicken recovery from mainline breakdowns back to or below the critical occupancy threshold. Typical results include reductions in travel times and crash rates, plus increases in traffic speeds.
To accelerate deployment of ramp metering across the country, the Federal Highway Administration initiated an effort to better understand the current state of the practice, identify reasons why ramp metering is not more widely deployed, and facilitate implementation by publishing guidance materials and conducting workshops.
Ramp Metering Components
Ramp metering requires three essential components and equipment to operate safely and effectively: signal heads, detectors, and signage.
Signal heads house the indicator lights and have either two or three sections. Two-section heads have green and red indications. Three-section heads include the yellow indication and might be more familiar to most drivers.
Detectors monitor conditions on the ramp and mainline. For example, the signal should turn green only when a vehicle is detected at the stop bar. Some installations also use queue detectors placed upstream of the stop bar to monitor and manage queue length. Detectors also can monitor demand on the mainline and merge points to aid in determining the metering rate. Detection equipment on the ramps themselves (such as a stop bar-located demand detector) should be connected directly to the ramp controller cabinet.
Signs should be placed at the start of the ramp and near the signal head. The sign content can provide instruction to vehicles when they enter the ramp and at the signal. Upstream signs with lights or adaptive screens also might help by indicating whether the ramp is being metered at a given time. Proper signage facilitates clear communication and compliance with the established system.
Ramp Metering Approaches
Depending on the existing infrastructure, constraints, or objectives of the ramp metering program, an agency may select various ramp metering approaches. The three primary types of strategies are fixed time, local control, and systemwide control.
Fixed time metering is the simplest approach in terms of implementation because it has no reliance on traffic detection or communication with a traffic management center. However, it is also the most rigid because it cannot make adjustments to the metering rate based on real-time mainline or ramp conditions.
Both systemwide and local control rely on loop detectors or other forms of traffic surveillance to select metering rates. A local control strategy will select metering rates based on traffic conditions present on the ramp and at adjacent mainline locations to remedy isolated congestion or safety-related problems. Local control cannot factor in conditions at adjacent ramps or throughout the freeway mainline. Local control is often used as a backup strategy when systemwide algorithms are offline or communications are inoperable.
Systemwide control is responsive to both local and corridorwide real-time traffic conditions, making a robust detection system critical for success. When calculating a metering rate, systemwide control takes into account traffic conditions upstream and downstream from an individual ramp along a specific freeway segment or along an entire corridor. Systemwide control provides more options for optimizing mainline capacity and reducing the amount of overall system delay by using multiple ramp meters to control traffic at any given bottleneck or congested location.
Other metering considerations include the number of lanes provided on the ramp, the number of vehicles released per cycle, the type of roadway the ramp connects to, and whether the road-way includes bypass lanes.
Single or multilane metering. Single lane metering allows only one vehicle to enter the freeway during each signal cycle. Multilane metering requires two or more lanes on the ramp and a signal head dedicated to each lane. After the stop bar, the lanes merge into a single lane before merging onto the freeway.
Single or dual release metering. One vehicle per green signal (single release metering) operates with a shorter green time than with metering that allows two vehicles per green (dual release). Dual release metering usually allows increased ramp capacity.
Freeway-to-freeway connections. Ramp metering on freeway-to-freeway ramps is less common because of the high travel speeds and the perceived increased potential for vehicle collisions because of vehicle queues where drivers may not expect them. Geometric constraints also exist, such as limited sight distance along a curved roadway and limited space for queued vehicles on ramps. Freeway-to-freeway metering, if possible, can significantly improve the ability to manage traffic on a freeway because a greater share of entering traffic is controlled by meters.
Bypass lanes. Bypass lanes enable a specific class of vehicle (usually a high-occupancy vehicle, a bus, or, in some locations, a truck) to avoid delay at ramp meters and have the right-of-way to merge directly on to the freeway. Bypass lanes may have a single detection loop, actuations of which could incrementally increase the length of the ramp meter cycle each time it detects a vehicle.
Ramp Metering Benefits
When agencies implement effective ramp metering programs using strategies suitable to the region, they often realize significant, long-term benefits. Although the magnitude of the benefits may vary depending on the level of congestion and configuration, common benefits persist across many regions. The widespread benefits of ramp metering, relative to its costs, make it one of the most cost-effective freeway management strategies.
“Proactively operating the transportation system with a strategy such as ramp metering is extremely cost effective,” says Jeffrey Lindley, associate administrator for FHWA’s Office of Operations. “Ramp metering can be implemented relatively quickly and inexpensively since it seeks to make more efficient use of existing capacity, rather than constructing new capacity.”
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To Ramp Meter or Not to Ramp Meter? When determining whether to deploy a ramp metering strategy, agencies must consider several things, including the following:
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Ramp metering reduces mainline congestion and overall delay, while increasing mobility through the freeway network and traffic throughput. Travel times, even when taking into account the time in queue on the ramp, are generally reduced. Travel time reliability is an important measure of the effectiveness of ramp metering.
Ramp meters break up platoons of vehicles that are entering the mainline, which facilitates smoother merging to help reduce collisions. Effectively managing ramp queues also can prevent queues from spilling onto the adjacent arterial and clogging up the city street network with stopped vehicles that are waiting to enter the freeway.
Ramp meters also help to eliminate prolonged periods of stop-and-go conditions caused by traffic congestion. Decreasing congestion can reduce vehicle emissions and fuel consumption. However, emissions and fuel consumption also may increase at the ramp meter. Environmental analysis must be sensitive to actual ramp operations and fuel estimation methodologies, especiallygiven the prevalence of electric and hybrid vehicles on the roadway.
| Examples of Ramp Metering Improvements by Performance Measure |
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| Safety Improvements | |
| Denver, CO | 50% decrease in rear-end and side-swipe collisions |
| Detroit, MI | 50% decrease in total collisions 71% decrease in injury collisions |
| Long Island, NY | 15% decrease in collision rate |
| Minneapolis, MN | 26% decrease in peak period collisions 38% decrease in peak period collision rate |
| Portland, OR | 43% decrease in peak period collisions |
| Seattle, WA | 34% decrease in collision rate |
| Travel Time/Speed Improvements | |
| Denver, CO | Increase in the average peak period speed from 43 to 50 miles per hour (69–80 kilometers per hour) |
| Long Island, NY | 9% increase in average vehicle speed |
| Minneapolis, MN | Increase in the average peak period speed from 40 to 43 mi/h (64–69 km/h) |
| Portland, OR | Increase in the average peak period speed from 16 to 41 mi/h (25–65 km/h) |
| Seattle, WA | Decrease in the average travel time from 22 to 11.5 minutes |
| Throughput Improvement | |
| Denver, CO | 18% increase in peak volume |
| Long Island, NY | 2% increase in throughput |
| Seattle, WA | 74% increase in peak volume |
| Environmental Improvement | |
| Minneapolis, MN | Savings of 1,160 tons (1,052 metric tons) of emissions |
| Source: FHWA, Ramp Management and Control Handbook (FHWA-HOP-06-001). | |
Ramp Metering Challenges
Although ramp metering has proven to be successful in many cities, departments of transportation continue to face barriers and opposition in their attempts to deploy or expand their ramp metering systems. In a survey that FHWA distributed to transportation agencies in highly populated metropolitan areas, the DOTs cited common barriers, such as a lack of support--whether financial or political--and existing infrastructure that is incompatible with ramp metering.
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 include inadequate acceleration length, mainline weaving problems because of closely spaced ramps, and limited sight distances on a horizontal or crest vertical curve. Ramps that are shorter in length or have less storage space are at a higher risk of arterial backup than longer ramps with similar demand. 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 and result in inefficient arterial operations.
A lack of sufficient funding for deploying and maintaining ramp metering systems is also a barrier for some DOTs. Without sufficient and well-timed funding, deploying ramp metering might be unsuccessful and not realize its full benefits. Agencies working to procure funding for ramp metering programs could face challenges if other initiatives have greater priority or support.
In addition, many DOTs also encounter public opposition that significantly deters or halts efforts to deploy ramp meters. Often this opposition is rooted in misconceptions about ramp metering and its effectiveness. The public might also raise concerns about equity, claiming that ramp metering could asymmetrically favor those who live outside the city center by giving those vehicles priority over the metered vehicles.
Deploying or expanding ramp metering systems often impacts a variety of stakeholders, including local agencies and businesses. These groups might have a negative perception of ramp metering and might have the resources to halt efforts to deploy or expand ramp metering. Local agencies often are concerned that the ramp queues will spill onto and block adjacent arterials, negatively affecting their transportation systems. Conversely, such groups could be valuable allies in building support for ramp metering and ensuring smooth operations.
Sometimes the agency with responsibility for freeway operations may not support ramp metering because it lacks understanding or knowledge of the strategy’s benefits; has concerns about long-term maintenance and operations costs; or does not have a sufficient number of staff with the skills and knowledge needed to implement or operate ramp metering.
Planning and Implementation
To address the challenges to ramp metering, FHWA published a primer titled Ramp Metering: A Proven, Cost-Effective Operational Strategy (FHWA-HOP-14-020) in October 2014. The publication outlines the following steps to plan and deploy a ramp metering system.
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Barriers to the Deployment of Ramp Metering In 2014, FHWA surveyed transportation agencies in highly populated metropolitan areas in the United States. Respondents cited these common barriers to deploying ramp metering:
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Full-Scale Field Test in Minnesota The Minnesota Department of Transportation (MnDOT) uses ramp meters to manage freeway access on approximately 210 miles (338 kilometers) of freeways in the Twin Cities metropolitan area. Although MnDOT has a long history of using ramp meters as a traffic management strategy, some members of the public questioned their effectiveness. A State bill passed in 2000 required MnDOT to study the effectiveness of ramp meters in the Twin Cities metropolitan area by conducting a field test to collect traffic data at specific ramps and along selected corridors within the region. The study compared several weeks of no metering with several weeks of fully metered freeway operations. The study, conducted in fall 2000 and published by MnDOT in 2001, determined the following:
For details, see the full report at www.dot.state.mn.us/rampmeter/pdf/finalreport.pdf. |
Step 1: Determine feasibility. An agency considering whether to deploy a ramp metering system should have an understanding of the high-level regional and agency goals, over both the short and long term. Deployment of ramp metering should contribute to and complement existing initiatives and adhere to high-level objectives, such as addressing issues of mainline congestion, safety issues at merge points and on the mainline, and impacts on construction and special events.
Before planning for any metering strategies, an agency should conduct feasibility studies and cost-benefit analyses to determine whether ramp metering is a practical strategy. Feasibility studies should involve examining the corridor’s existing traffic conditions and ramp capacities. DOTs can utilize FHWA resources and tools to identify capital and operating costs related to the installation of new systems. An agency can strengthen its case for deploying ramp metering by itemizing specific costs and providing cost-benefit information. The DOT should also assess its own internal capacity to successfully implement and operate ramp metering.
Step 2: Plan around policies and agency needs. DOTs should plan for ramp metering based on regional policy and recommended outcomes from feasibility studies. This planning includes assessing staffing needs, organizational capabilities, and agency readiness, as well as planning for data collection, funding procurement, user needs, system requirements, and equipment testing.
Even prior to making a decision to meter a corridor, agencies can preempt infrastructure challenges by building ramps with consideration for metering. In other words, when an interchange construction project occurs on a non-metered corridor, agencies could install elements like proper geometry, conduits, and detectors to support a later application of metering if it is deemed necessary.
Step 3: Gain public and agency support. For both agencies and the public, one of the best ways to encourage support is by proactively disseminating information and clearly communicating the benefits of ramp metering. To gain public favor, DOTs should make substantial efforts to educate and involve the public in their plans--beginning at least 3 years prior to activating the first ramp metering system in a metropolitan area, if possible. Outreach may include inviting and gathering public feedback, establishing a speakers’ bureau for presenting educational material to stakeholders, and issuing statements to local media. Efforts also might include maintaining a project Web site and distributing information via brochures and flyers. Outreach should continue through the first year of deployment.
Presenting the results of a cost-benefit analysis to improve communication is key to alleviating these challenges, as is measuring and presenting performance metrics. In addition, internally communicating early and often and during the planning process is key to successful deployment.
Step 4: Update costs and secure funding sources. Agencies should next make any necessary updates to the capital and operating cost estimates and cost-benefit information developed during feasibility studies. To secure funding, agencies also must communicate the high priority of ramp metering to authorities.
Step 5: Implement ramp metering. Implementation involves systems engineering and project design, as well as the actual installation and operation of ramp meters. FHWA has resources to help agencies through these processes for ramp metering.
Step 6: Manage queues and delays. Managing queues is crucial for optimizing freeway operations, and long queues generally contribute to negative public perceptions. The DOTs should develop policies on maximum queue wait times and be able to communicate and justify the policies publicly.
Step 7: Measure and report performance. The DOTs should establish which performance metrics to measure, appropriate benchmarks for those measures, monitoring procedures, and reporting procedures. Metrics might include safety, mobility, public acceptance, travel time reliability, facility throughput, and environmental impacts. In connection with step 6, agencies should give particular attention to the length and behavior of the ramp queues and establish policies and methods for regularly monitoring them, as traffic conditions can change over time and throughout the day.
Going Above and Beyond
After deploying a basic ramp metering system, the operation of the system might benefit further from enhancement. The extent to which an agency can and should develop its ramp metering program varies greatly, depending on factors such as agency policy preferences and management of excess traffic volume. Through ongoing performance monitoring and internal assessment, DOTs can identify and plan for enhanced ramp metering strategies in future regional planning. Operational enhancements might include policies that extend the use of ramp meter operation outside peak hours and for special events and construction activities, special ramp treatments, adaptive ramp metering, and integrated freeway and arterial corridor management strategies.
For example, in November 2012, the Washington State DOT automated the operation of ramp meters in Seattle by adding a function to the algorithm that looks at traffic conditions locally and downstream of the ramp. The meters now turn on in response to real-time traffic conditions, rather than waiting for an operator at the traffic management center to observe congestion and manually activate the system.
Special ramp treatments include strategies that can improve traffic conditions, increase safety at merge points, and provide driver incentives for specific modes of travel. For example, to encourage carpooling and transit use, some ramps have high-occupancy vehicle bypass lanes for those vehicles to avoid delay when merging onto the freeway.
Adaptive ramp metering is another possible enhancement. This technique is effective for recurring and nonrecurring congestion, because it responds to mainline traffic conditions and on/off mainline flow in real time.
Another possible enhancement to a ramp meter system is integration of an arterial signal system as part of a systemwide strategy for corridor management. When operating independently of ramp meter signals, arterial signals may release too many cars onto the ramp, causing backup onto the arterial. However, if the two systems are integrated, it can lead to safer and more efficient conditions.
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Lessons Learned in the Field FHWA engaged practitioners currently operating ramp metering systems to share their experiences and lessons learned. The following are their top recommendations:
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These approaches tend to require high organizational capability and are not necessarily suitable for all ramp metering locations. “Even agencies with existing ramp meters should assess both suitability and feasibility prior to expanding and improving their programs,” says Robert Arnold, director of the FHWA Office of Transportation Management.
Successful operation of ramp metering systems can lead to greater integration with other activities, such as road weather management and incident management, that actively manage the freeway network. To provide optimized system performance, DOTs need to actively and continuously manage their transportation networks. The end result is a program that will improve mobility, reliability, safety, and environmental impacts while preserving freeway capacity--at a significantly lower cost than traditional capacity improvements.
James Colyar, P.E., is a transportation specialist on the Systems Management Team at the FHWA Office of Operations. He has more than 15 years of experience in traffic engineering, traffic analysis and modeling, and transportation systems management and operations. Colyar holds a B.S. in civil engineering from the University of Idaho, an M.S. in civil engineering from NC State University, and an M.A. in transportation, policy, and logistics from George Mason University.
Rachel Klein is a research scientist on the Critical Infrastructure Transportation Operations team at Battelle Memorial Institute. She has more than 7 years of experience in transportation demand management, transportation systems management, and transportation research. Klein holds a B.S. in civil engineering from the University of Maryland, and an M.S. in transportation systems engineering from Cornell University.
Les Jacobson, P.E., is a vice president and senior intelligent transportation systems manager for Parsons Brinckerhoff.He has supported domestic and international agencies in developing and operating ramp metering systems for more than 35 years. Jacobson holds a B.S. in civil engineering from the University of Washington and an M.S. in civil engineering from the University of California, Berkeley.
For more information, see www.ops.fhwa.dot.gov/freewaymgmt/ramp_metering/index.htm or contact James Colyar at 360–753–9408 or james.colyar@dot.gov. If you are interested in hosting an FHWA-sponsored ramp metering workshop, contact James Colyar.