Evaluation of variable speed limits for real-time freeway safety improvement

Use of various variable speed limit (VSL) strategies as a tool for safety improvement on freeways was evaluated using simulation of a section of Interstate 4 in Orlando, FL. Real-time crash likelihood was calculated based on models developed in previous research by the first author [Abdel-Aty, M., Uddin, N., Pande, A., January 2005. Split models for predicting multi-vehicle crashes during high-speed and low-speed operating conditions on freeways. In: Presented at the 84th Annual Meeting of the Transportation Research Board, Washington, DC]. VSL implementation produced safety improvement by simultaneously implementing lower speed limits upstream and higher speed limits downstream of the location where crash likelihood is observed in real-time. This improvement was realized in the case of medium-to-high-speed regimes on the freeway, but no benefit was achieved in low-speed situations (no substantial safety benefit from implementing VSL in congested situation's simulation). The final recommendations for implementing VSL are:
• gradually introducing speed limit changes over time (5 mph every 10 min);
• abruptly changing speed limit in space (no gap distance);
• reducing speed limits upstream and increasing speed limits downstream of location of interest;
• the speed limit changes up- and downstream should be large in magnitude (15 mph) and implemented within short distances (2 miles) of the location of interest.In addition to the safety benefit, this final strategy also produced travel time savings.