An Analysis of Threshold Criteria and Conditions for Variable Speed Limit Deactivation

Abstract

The study demonstrates the relative effectiveness and trade-offs amongst various threshold conditions for Variable Speed Limits (VSL) deactivation in terms of the control stability and efficiency. Reduced speed limits must be restored when the traffic interruption is resolved in a prompt and safe manner. Most VSLs in practice rely on a threshold-based control, which determines the speed limit adjustment by comparing detector readings to pre-determined threshold conditions. We tested three types of algorithmic parameters including occupancy, average speed, and consistency-check conditions. The test was carried out using the actual detector data, collected from three different segments of Pacific Motorway in Brisbane, Australia. Our analysis revealed that occupancy could be the most effective threshold over average speed. Operational fluctuation of VSL can be significantly reduced by selecting appropriate occupancy threshold values. We also tested four different consistency check conditions including a conditional deactivation approach. This approach makes the deactivation decision conditional upon the traffic status in the immediate downstream segment to avoid flushing upstream traffic into an active or potential bottleneck downstream. The test results and findings of this study may be useful for practitioners and researchers for developing VSL deactivation rules and associated operational strategies.