What is ramp metering

Queue management is crucial for optimizing freeway operations, and long queues generally contribute to negative public perception. Queue management is closely related to the geometric limitations of the existing ramps, so both could be considered when exploring options.

Ramp metering technology has developed various methods of queue detection and corresponding solutions, such as being hard-wired into the controller or with the software as a function of the control algorithm. If adding storage capacity at a ramp is not feasible, an agency could consider adjusting the metering control scheme to increase ramp metering capacity.

Agencies should create policies on maximum queue wait time, and be able to communicate and justify these policies publicly. San Diego had insufficient storage on many of its ramps, but the local agency collaborated with agencies responsible for arterial signals to reduce the impact of long queues on arterial traffic. The agency should study pre-metering traffic conditions of the corridor.

The current and anticipated mainline volumes, ramp volumes, and surrounding arterial volumes should be evaluated in regards to how they will be impacted by ramp metering. The capacity of the ramps will likely influence the other aspects of the ramp meter operations.

After turning on the ramp meters, agencies should determine their impact on operations, identify areas of improvement, and continue building inter-agency and public support. By building a robust performance assessment process, agencies can learn about their current system and whether it should be expanded. Agencies should establish which performance metrics to measure, appropriate benchmarks for those measures, monitoring procedures, and reporting procedures.

Dimensions of performance metrics to measure can include safety, mobility, public acceptance, travel time reliability, facility throughput, and environmental impacts.

Measuring the costs and benefits of a proposed ramp metering system is a practiced strategy for accelerating ramp metering deployment. Agencies could face scrutiny regarding the proposed benefits of ramp metering in their regions. Agencies should give particular attention to the length and behavior of the ramp queues and establish policies and methods for regularly monitoring the ramp queues, as traffic conditions can change overtime and throughout the day.

Other, smaller cities in our state have seen the benefits and are planning ramp meter systems. Agencies considering expansion or further development of their ramp metering program should first assess the current state of the program. In particular, the agencies should evaluate current performance metrics in order to target areas of opportunity and set expectations for improvement.

The impact of the proposed expansion should be reported and shared with relevant stakeholders. In a FHWA survey of the top metro areas, there was a high level of interest in expansion amongst areas that had already deployed ramp meters. Select environmental impacts: Minneapolis identified a net annual savings of 1, tons of emissions Note: emission values calculated based on a simplified approach on average changes in speed. Other simulation analysis shows ramp metering to be effective in reducing CO emissions by 1, tons per year.

Overview of Ramp Metering Due to the growth of metropolitan areas and tightening of fiscal belts, the need for effective and financially viable freeway management tools is unprecedented. Figure 1: Ramp metering configuration Source: Parsons Brinckerhoff.

Mobility, Reliability, and Efficiency Ramp metering reduces mainline congestion and overall delay, while increasing mobility through the freeway network and traffic throughput. Safety Ramp meters help break up platoons of vehicles that are entering the freeway and competing for the same limited gaps in traffic.

Reduced Environmental Impacts Ramp meters smooth the flow of traffic entering the freeway so vehicles can merge with mainline traffic with minimal disruption to traffic flow. Figure 5: Comparison of mainline conditions with and without ramp metering Source: Washington State Department of Transportation.

Figure 6: Duration of occupancy at shutdown capacity before and after ramp metering Source: Parsons Brinckerhoff. The following is a high-level overview of commonly used control approaches for ramp metering: Single or Multi-Lane Metering— Single lane metering allows only one vehicle to enter the freeway during each signal cycle.

Detection in the field is not needed. Requires periodic manual updates. Not effective for non-static conditions. Higher operations costs compared to traffic responsive systems. Appropriate for localized problems. Detection in the field is needed. Higher capital and maintenance costs compared to pre-timed systems. Yields greater benefits because it responds to conditions in the field. Appropriate for widespread problems. Rarely used compared to system-wide, traffic responsive systems.

Most useful for corridor, system-wide applications. Greatest capital and maintenance costs, but yields most benefits. Metering rates are determined based on occupancy data collected from mainline loop detectors located downstream from the meter. The goal is to maximize the mainline throughput by maintaining occupancy values below the target threshold.

Metering rates determined based on mainline speeds and prevailing local controller conditions which are then optimized over each freeway direction. The concept is based on the assumption that a ramp can allow maximum vehicles when the speed is high and should allow no vehicles when the speed is near optimal. When ramp queues meet threshold conditions, the control algorithm is activated and assigns restrictive metering rates to upstream ramps in order to balance downstream conditions.

This algorithm incorporates ramp queues and makes them integral to managing and controlling the freeway system. This algorithm controls multiple ramps, uses more comprehensive mainline and ramp inputs, and uses different heuristics which allows for the most flexibility on determining metering rates for changing local conditions. This algorithm operates on density measurements and requires detection upstream of the ramp merge, at mainline exit ramps, and on the mainline to sustain the overall objective of the total volume exiting a zone exceeding the volume entering.

SZM attempts to rebalance the increase in mainline density by making other meter rates in the active zone more restrictive. Ramp queue wait times are managed by a separate algorithm function. Metering rates are calculated based on the current density, the required density, and the number of vehicles that should be removed or added to the freeway zone between each ramp.

Additional upstream ramps are called to action when a single ramp has exceeded its capacity to balance the zone density. SWARM provides advantages to mixed ramp controls, so that some ramps can be operated locally, manually, or with a different algorithm and SWARM will adapt to distribute wait times equally among ramps. When heavy traffic uses an on-ramp without a ramp meter, multiple vehicles try to merge at the same time. Drivers on the freeway now have to adjust to multiple vehicles, rather than just one at a time.

When drivers try to accommodate multiple vehicles entering the freeway by slowing down, they slow everyone else behind them down, quickly growing into a backup and congestion.

Freeway drivers may also slow down too much or too fast or be uncertain what to do, which could result in a collision. When an on-ramp is metered, only one vehicle is trying to merge, which is more comfortable for drivers on the freeway to accommodate.

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.

While effective ramp metering does not eliminate traffic congestion, it does delay the onset of congestion, as well as shorten the duration i. Better organized zipper merging and less stop-and-go traffic can mean fewer collisions, making freeways safer.

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 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.

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. Home category operations supporting infrastructure ramp metering Ramp metering.