A On The Ramp
Due to the growth of metropolitan areas and tightening of fiscal belts, the need for effective and financially viable freeway management tools is unprecedented. This primer poses ramp metering as a potential tool to address commonly occurring congestion and safety issues. Despite initial opposition and skepticism from various stakeholders, ramp metering has been deployed, sustained, and even expanded in many regions. This primer incorporates recent research on challenges agencies experience during their attempts to deploy or expand ramp metering in their regions.
a On The Ramp
While geometric limitations of existing ramps are a common challenge, agency support and project costs also pose difficulties for several agencies. Recent case studies provide insights into how these common challenges could be addressed as well as lessons learned. This primer emphasizes organizational capability, public outreach, and geometric limitations as key considerations when deploying or expanding ramp metering.
Ramp meters are traffic signals installed on freeway on-ramps to control the frequency at which vehicles enter the flow of traffic on the freeway. Ramp metering reduces overall freeway congestion by managing the amount of traffic entering the freeway and by breaking up platoons that make it difficult to merge onto the freeway. As seen in Figure 1, vehicles traveling from an adjacent arterial onto the ramp form a queue behind the stop line. The vehicles are then individually released onto the mainline, often at a rate that is dependent on the mainline traffic volume and speed at that time. The configuration in the diagram is the most common; however, some agencies have altered this design to accommodate transit and high-occupancy vehicle (HOV) policies or existing geometric limitations.
Ramp metering is one of many strategies in the realm of freeway management and operations that agencies use to operate the existing freeway network at full potential. Successful operation of ramp metering systems leads toward integration with other activities that actively manage the freeway network. Transportation Systems Management and Operations (TSM&O) is an integrated program, with strategies such as ramp metering, road weather management, and incident management (Figure 2), that requires continuous and active management by agencies to ultimately provide optimized system performance to existing freeway infrastructure. 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. Ramp metering can also support regional congestion management processes.
Ramp metering was first deployed in the 1960s on the Eisenhower Expressway in Chicago. In the subsequent years, ramp meters were deployed in major metro areas such as Detroit, Los Angeles, and Minneapolis/St. Paul as experiments in increasing driver speeds during peak travel periods while reducing travel times and the frequency of freeway crashes. As ramp metering strategies and techniques advanced, more metro areas across the U.S., as well as Europe and Australia, began implementing ramp metering systems.
The uniqueness of the cities furthered the advancement and refinement of ramp metering as a traffic management strategy. The different agency needs and priorities resulted in various strategies, such as preferential treatment for HOVs and transit through the designation of bypass lanes, special ramp treatments like metering multiple lanes (including ramp shoulders), and metering two or three vehicles per green. As displayed on Figure 3, ramp metering has been deployed in varying degrees of sophistication and scale across the U.S.
When agencies implement effective ramp metering programs using strategies suitable to the region, they often realize significant, long-term benefits. While 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.
Ramp metering reduces mainline congestion and overall delay, while increasing mobility through the freeway network and traffic throughput. Travel times, even when considering time in queue on the ramp, are generally reduced when ramp metering is implemented. Travel time reliability has become an important measure of ramp metering effectiveness. Many regions have experienced increased travel time reliability (reduced variations in day-to-day travel times) due to ramp metering.
Ramp meters help break up platoons of vehicles that are entering the freeway and competing for the same limited gaps in traffic. By allowing for smooth merging maneuvers, collisions on the freeway can be avoided. Many regions have reported significant reductions in crash rates after starting ramp metering.
Ramp meters smooth the flow of traffic entering the freeway so vehicles can merge with mainline traffic with minimal disruption to traffic flow. Eliminating prolonged periods of stop-and-go conditions due to congestion can reduce vehicle emissions and fuel consumption on the freeway. It can be argued that emissions and fuel consumption increase at the ramp meter, which is why the environmental analysis must be sensitive to actual ramp operations and fuel estimation methodologies, especially with the prevalence of electric and hybrid vehicles on the roadway.
Without ramp meters in operation, multiple vehicles merge in tightly packed platoons, causing drivers on the mainline to slow down or even stop in order 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 can break up the platoons by controlling the rate at which vehicles enter the mainline from the ramp, as shown on Figure 5. This allows vehicles to merge smoothly onto the mainline and reduces the need for vehicles on the mainline to reduce speed. In addition to breaking up platoons, ramp meters help manage entrance demand at a level that is near the capacity of the freeway, which prevents traffic flow breakdowns. Ramp meters are shown to reduce peak hour lane occupancies (i.e., freeway density) and quicken recovery from mainline breakdown back to or below the critical occupancy threshold, as shown on Figure 6. Typical results include reductions in travel time, reductions in crash rates, and increased traffic speed.
The technology utilized by ramp meters has evolved to offer increased scalability, efficiency, and customization. Through developed simulations and databases, agencies can model the effects of ramp metering on their existing freeway operations. These exercises can aid in the decision-making and planning process when planning for ramp metering deployment or expansion.
Simulation models can aid in determining what control algorithm would be most appropriate to manage freeway congestion based on user-defined inputs on system conditions. Computer-based simulations can estimate a set of traffic measures, such as travel time speed, for a given freeway corridor. The simulations can use new operational strategies like adaptive ramp metering coded directly into the software so that realistic environments can be evaluated for a proactive approach to congestion management.
Optimal ramp meter operations require robust data collection and analysis. Data regarding traffic volumes, travel times, and other appropriate performance measures should be collected, modeled, and analyzed both before and after ramp meters are installed. Volume data is collected in the form of video or loop detection along the ramp and on the freeway mainline upstream and downstream of the ramp.
An integrated database, that contains data such as roadway inventory, detector data, traffic counts, crash records, and incident records, can streamline the process to evaluate the effectiveness of various ramp metering strategies or algorithms. The evaluation database not only reduces the effort to collect all these data sets independently, but can be used with enhanced data sets and could provide the data needed to support visualization of various ramp metering scenarios.
Depending on the existing infrastructure, constraints, or objectives of the ramp metering program, an agency may select various ramp metering approaches. Table 1 provides details on various levels of control, including appropriate situations to use each approach. The following is a high-level overview of commonly used control approaches for ramp metering:
Ramp metering requires some essential components and equipment in order to operate safely and effectively. Configuration, communication, and safety are objectives agencies should consider when placing signal heads, detectors, and signs. The following are key components in most ramp metering setups:
There are a wide range of ramp metering control strategies and algorithms. The three primary types of control strategies are fixed time, local control, and system-wide 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 since it cannot make adjustments to the metering rate based on changing real-time mainline or ramp conditions. Both system-wide and local traffic responsive 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 back-up strategy when system-wide algorithms are offline or communications are inoperable. System-wide control is responsive to both local and corridor-wide real-time traffic conditions. When calculating a metering rate, system-wide control takes into account traffic conditions upstream and downstream from an individual ramp along a specific freeway segment or along an entire corridor. System-wide control provides more options in optimizing mainline capacity and reducing the amount of overall system delay by using multiple ramps to control traffic at any given bottleneck or congested location. Descriptions of example algorithms are included in Table 2.