| The use of variable speed limits (VSL) permits the posted speeds to be determined based on current conditions and to vary by time and location within work zones. This research evaluated the effectiveness of the VSL system under various work zone conditions based on data collected during a field test in work zones in Michigan. The measures of effectiveness used in this evaluation included changes in average speed, travel time through the work zone, 85 th percentile speed, speed variance, and percentage of higher speed vehicles. MOE data “before,” “during,” and “after” the VSL operation were compared.; The results indicated that the VSL system worked well at locations in the work zone where there were no ramps in the vicinity. Relatively large increases in the displayed speeds (8–10 mph) at these locations resulted in small increases in the average speeds (1–2 mph), which in turn resulted in the reductions in the travel times through the work zone. The percentage of higher speed vehicles at most of the locations either stayed the same or became lower “during” the VSL deployments, which indicates an improvement in speed limit compliance. However, at most trailers the 85 th percentile speeds did not change in the same direction as the changes in the displayed speeds as hypothesized. The impact of the system on speed variance was inconsistent.; Although the changes in the average speed and the travel time following the deployment of VSL were small, they were in the right direction. The VSL did respond to the day-to-day and trailer-to-trailer average speed variations, and thus the credibility of the posted speed limit was improved. |
