A Northern Virginia experience
In July 2008, a variable speed limit (VSL) system was implemented on a 7.5 mile section of I-495 (the Capital Beltway) in Virginia between the Springfield Interchange and the Woodrow Wilson Bridge. A study was conducted to evaluate performance, however, after the system was implemented changes to site conditions precluded a direct comparison of before and after data, thus a simulation model (VISSIM) was constructed to estimate system impacts.
VISSIM simulated traffic activity based on traffic volume data provided by the VDOT Traffic Monitoring System. Several measures of effectiveness (MOEs) were evaluated. Travel time, speed, queue length, number of stops, and lane change information were collected throughout the simulation. Travel time and mean speed served as operational measures, and the standard deviation of speed, mean queue length, and mean number of lane changes (per 5 minute interval) served as surrogate measures of safety. The model was calibrated using field data collected from floating car runs made between June and September 2008. Researchers identified where vehicle speeds dropped as a result of work zones and made adjustments to the model to match real-world travel times within 10 percent.
Overall, the system implemented on the Capital Beltway was designed to harmonize upstream and downstream traffic flow around each work zone. The work zones were configured with tapered lane closures that reduced travel lanes from 4 lanes to 2 lanes or from 4 lanes to 1 lane. In addition to calculating optimal speeds, the VSL system was configured to limit the frequency of speed limit changes. Previous research suggested that 2-minute intervals between posted speed limit changes would be detrimental to safety, whereas 5- or 10-minute intervals would reduce crash potential (Lee, et.al., 2004).
Seven VSL signs were installed between the Springfield Interchange and the US Route 1 Interchange on the Outer Loop. Five VSL signs were installed between the Maryland-Virginia border on the WWB and the Eisenhower Connector Interchange on the Inner Loop. The total cost of the system was $3.2 million over two years including hardware, software, training, and operational support.
VSL system (hardware, software, training, and operational support included): $3.2 million.