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).
Benefits were computed by examining differences in travel time between the base case with static speed limits and the best performing VSL alternative. The simulation model indicated that the system can improve traffic conditions if the demand does not exceed capacity by too large a margin. For example, in the case of the 4-to-1 lane closure, none of the VSL alternatives evaluated produced an increase in network speeds. However, in the case of the 4-to-2 lane closure, the best VSL configuration resulted in a mean savings of 267.04 vehicle-hours of delay, which was translated into $12,229.76 user delay savings per day in the control area when the lane closure was present.
(Our website has many links to other organizations. While we offer these electronic linkages for your convenience in accessing transportation-related information, please be aware that when you exit our website, the privacy and accessibility policies stated on our website may not be the same as that on other websites.)