Appendices : evaluation of variable speed limits on I-270/I-255 in St. Louis.

Bahm, Ghulam; Long, Suzanna; Baik, Hojong; Ryan, Tom; Gentry, Lance; Lall, Khushboo; Arezoumandi, Mahdi; Liu, Daxiao; Li, Tao; Schaeffer, Brian · 2010 · ROSA P / Missouri. Department of Transportation

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Summary

This report evaluates the performance of a Variable Speed Limit (VSL) system installed on the I-270/I-255 corridor in St. Louis, Missouri, in May 2008. As the first such implementation in the state, the study aimed to assess the system’s impact on traffic mobility, highway safety, and stakeholder perceptions. The evaluation compared pre-VSL baseline data with post-VSL data to determine if the dynamic speed limits effectively managed recurring and non-recurring congestion. The methodology focused on four specific congested segments of I-270, analyzing data collected via Remote Traffic Microwave Sensors (RTMS) and GPS field runs. The researchers employed both uncontrolled analysis, which included all weather conditions and congestion types, and controlled analysis, which isolated days with similar traffic volumes and clear weather to isolate the VSL’s effect. Key performance measures included volume, occupancy, average speed, speed compliance, highway capacity, and travel time reliability indices (Travel Time Index, Buffer Time Index, and Planning Time Index). Additionally, the study assessed safety through crash data and gathered qualitative feedback from the public and law enforcement regarding system visibility and compliance. The findings indicate that the VSL system did not achieve its primary goal of improving overall mobility along the corridor. While limited benefits were observed in specific segments, the system failed to significantly reduce congestion or delay. However, the evaluation identified noticeable safety benefits, specifically a reduction in the number of crashes during the evaluation period. Despite these safety gains, the study found widespread dissatisfaction among drivers and law enforcement. Stakeholders reported low perceived benefits for congestion relief, poor compliance with posted limits, and issues with the visibility of the current sign configuration. The analysis also highlighted discrepancies between detector-based data used by the VSL system and actual GPS travel runs, suggesting that the system’s operational logic may not accurately reflect real-time traffic conditions. The significance of this study lies in its comprehensive assessment of Active Traffic Management systems in a real-world setting. It demonstrates that while VSL systems can contribute to highway safety by reducing crash frequencies, they may not effectively alleviate congestion if driver compliance is low or if system logic does not align with actual traffic flow dynamics. The report underscores the importance of stakeholder perception and system visibility in the success of traffic management technologies, providing critical insights for future implementations and potential enhancements to VSL operations.

Key finding

The Variable Speed Limit system resulted in a noticeable reduction in crashes but failed to improve overall mobility along the corridor while facing widespread dissatisfaction from the public and law enforcement.

Methodology

field_study

Provenance

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Topics

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• speed management
• speed choice
• traffic density