Assessment of Variable Speed Limits from the drivers´ perspective

Evaluating of Variable Speed Limits (VSL) is often only possible in indirect ways due to the lack of sufficient data. Since the majority of safety benefits is expected from warning and increased alertness of drivers, evaluation methods that focus on the informative nature of VSL such as the detection rate (DR) and false alarm rate (FAR) are useful tools. In the state-of-the-art these are typically defined from the system´s perspective. In this paper, an assessment based on DR and FAR defined from the drivers´ perspective is presented, using vehicle trajectories integrated from speed data, hereby evaluating the performance of the VSL by the effectiveness of warning drivers. Appropriate visualization of DR and FAR over traffic speed contours in space and time allows identifying locations or traffic conditions in which missed detections or false alarms are concentrated, so that targeted measures for improvement can be taken. Using the presented method, the VSL system around Antwerp (Belgium) is evaluated. Suggestions for improvement of safety and credibility are highlighted and evaluated using the assessment from the driver perspective.