Evaluation of On-Ramp Signals on Interstate 540 in North Carolina Emphasizes the Importance of Arterial Signal Timing Coordination.
On-ramp signals or ramp meters are used to control the number of vehicles allowed to enter a freeway to allow for smoother merging at on-ramps. North Carolina Department of Transportation (NCDOT) commissioned a study to assess the first-year operational impacts of four on‐ramp signals which were installed along Interstate 540 in North Carolina in September 2017. Signals were positioned on the westbound lanes of four on-ramps in Wake County and have been regularly activated Monday to Friday from 6:30 ‐ 9:00 am. Researchers analyzed the impacts to traffic using data collected in the affected area from numerous sources, both before and after the installation of on‐ramp signals. When feasible, the costs and benefits of the on‐ramp signals were monetized using benefit cost analysis methods.
- Arterial signal timing should be considered for optimal results. Signals surrounding the on‐ramp signals should use time of day plans that are re‐evaluated and re‐timed as needed every three to six months in the first 1.5 years of on‐ramp signal operations.
- Quantity and diversity of data collected is important. It is essential to collect as much traffic data as possible before on-ramp signal installation and ensure that the same data collection methods are applied after installation. Similarly, collecting diverse types of data, like traffic volumes and travel times is key to correcting errors and conducting sensitivity analyses for improved accuracy.
- The locations selected for data collection matter. Count and turning movement data should be collected at multiple points throughout the mainline, adjacent arterials that cross an on-ramp signal area, and any roadway that may be used as an alternate route. It is suggested that video monitoring data be collected near on-ramps to allow for visual confirmation of traffic data results.
- Timing of data collection should be planned carefully. Ideally, baseline data for the corridor should be collected at least a full year prior to on‐ramp signal installation, as traffic conditions will vary by season.
- Design and placement of detectors should be well thought-out. Ensure that the location of the lane drop arrow is farther downstream of a potential meter stop bar. Placing intermediate queue detectors on the ramps may improve the overall effectiveness of the on‐ramp signal system, as they can provide additional data that can improve operations.
- Adequate staffing and operations are essential, especially early-on in the implementation. For the first two to three months of a project, staff should document all changes to on‐ramp signal operations, outages, and other variables which may influence regular traffic operations.