Evaluation of Four Control Algorithms Used in Variable Speed Limit Systems

Control algorithms used for deciding on the speed limits in variable speed limit systems are crucial for the performance of the system. Today, many of the control algorithms used are based on fixed thresholds in speed and/or flow for lowering and increasing the speed limit. The algorithms are not necessarily reflecting the conditions on the road, which might lead to low traffic efficiency. Our hypothesis is that by use of a simple and efficient control algorithm that is better in reflecting the conditions on the road, both traffic efficiency and traffic safety could be increased. In this study, four control algorithms used in variable speed limit systems, and fulfilling the above criteria, are evealuted through microscopic traffic simulation. Performance indicators related to traffic safety, traffic efficiency and environmental impacts are presented. The results show that the design of, and the objective with, the control algorithm have a great impact on the performance. Moreover, the time needed for incident detection, the duration of and the size of the speed limit reduction and the location of the congestion are of importance for the performance of the control algorithms. These results will be of importance for design and implementation of future efficient variable speed limit systems.

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