Ramp Metering: A Proven, Cost-Effective Operational Strategy—A Primer
4. Going Above and Beyond
After deploying a basic ramp metering system, the operation of the system may benefit further from increased sophistication. The extent to which an agency can and should develop its ramp metering program can vary greatly. The need for these enhancements can also vary greatly, from agency policy preferences to managing excess volume. One of the key attitudes that agencies with successful ramp metering programs embrace is the desire for continued improvement in ramp metering operations. Through ongoing performance monitoring and internal agency assessment, enhanced ramp metering strategies can be identified and planned for in future regional planning efforts. Operational enhancements can include policies that extend the use of ramp meter operation outside the peak hours and for special events and construction activities, special ramp treatments, adaptive ramp metering, and integrated freeway and arterial corridor management strategies.
Special ramp treatments include strategies that can improve traffic conditions, improve safety at the merge point, and provide driver incentives for specific modes of travel. For instance, to encourage carpooling and transit use, some ramps have HOV bypass lanes that allow HOV and transit vehicles to avoid delay by bypassing the ramp queue and merging onto the freeway.
WSDOT: Automated turn on/off
In November of 2012, WSDOT automated the on/off operation of the Seattle area ramp meters through an added function to the Fuzzy Logic algorithm that looks at traffic conditions local and downstream of the ramp. All meters have the ability to be automated and most of them are. The few that are not automated are still in manual because of presence of construction activity.
ODOT: Adaptive ramp metering
Oregon Department of Transportation (ODOT) switched its ramp meters from pre-timed operation to the SWARM algorithm in 2005 to provide increased efficiency and effectiveness of a traffic-responsive ramp metering algorithm. About 150 ramps in the Portland metropolitan area are under adaptive control during the morning and evening peak periods.
(See Table 2 for algorithm descriptions)
Adaptive ramp metering is one application out of the suite of Active Transportation and Demand Management (ATDM) concepts that aims to control the rate of vehicles entering the freeway utilizing advanced technologies. Part of the ATDM (and TSM&O) philosophy includes monitoring the transportation system and taking steps to continuously improve its operation. In other words, it is important to actively manage each ramp metering system. Exploration of basic ramp metering approaches to meet freeway management goals and objectives is beneficial prior to implementing more advanced technologies and concepts.
Agencies can also focus on developing the systems aspect of ramp metering. For instance, adaptive ramp metering employs algorithms that change release rates based on the level of congestion on the mainline. This technique is effective for recurring and non-recurring congestion, since it can respond to mainline traffic conditions and on/off mainline flow in real time.
Another enhancement to a ramp meter system may be the integration of the arterial signal system as part of a system-wide corridor management strategy. When operating independently of the ramp meter signals, the arterial signals may release too many cars onto the ramp, causing backup onto the arterial. If the two systems are integrated, backup could be reduced leading to safer and more efficient conditions.
These approaches tend to require high organizational capability and are not necessarily suitable for all ramp metering locations. As stated above, even agencies that already have ramp meters should assess both suitability and feasibility prior to expanding and improving their programs.