Study Details
Safety Impacts of Regulatory Variable Speed Limits
Study ID: 705
Authors: Meghna Chakraborty and Md. Shakir Mahmud
Publication Date: AUG, 2024
Abstract: Increasing the roadway capacity to accommodate higher traffic volume may not be a solution on many urban corridors and this limitation has been a significant safety concern. Particularly, roadways connecting major downtown areas typically experience increased crashes due to limited capacity. Active traffic management (ATM) systems have been found to be an effective countermeasure to reduce operation-related crashes. But, one particular ATM, namely variable speed limit (VSL), has limited use nationwide and was not studied to the extent required to determine its safety effectiveness. To this end, this study evaluated the safety effects of VSL in congested urban areas by analyzing Interstate 5 (I-5) and State Route 520 (SR-520) corridors in the greater Seattle region, Washington, where VSL was implemented in 2010. A before-after Empirical Bayes analysis was performed considering six years of crash data between 2007 and 2013, excluding 2010 when the treatment was applied. Separate models were developed for total, fatal and injury, and property damage only crashes on I-5 and SR-520. The results of this study overall indicate that the implemented VSL system reduced crashes across all severities and showed a more pronounced impact on I-5 compared to its SR-520 counterparts. Also, the higher decrease in fatal and injury crashes may be indicative of the effectiveness of VSL system in harmonizing the operating speeds. Overall, the findings from this study showed VSL to be an effective countermeasure in reducing crashes of all severities on congested urban corridors and contributed to the limited knowledge on the safety impacts of automated regulatory VSL systems.
Study Citation: Chakraborty M., and Mahmud M. S. 2024. Evaluation of the Safety Impacts of Regulatory Variable Speed Limits. Institute of Transportation Engineers (ITE) Journal, pages 41-46.
Study Type: Journal Article/Book
Study Report: Download the Study Report Document
Comments: This study relies on secondary data that lack several crucial information including direction-specific data. So, this analysis could not control for directional information as well as account for temporal variability.
CMFs Associated With This Study
Category: Advanced technology and ITS
Countermeasure: Install Variable Speed Limit (VSL)
| CMF | CRF(%) | Quality | Crash Type | Crash Severity | Roadway Type | Area Type |
|---|---|---|---|---|---|---|
| 0.904 | 9.6 |  | All | All | Principal Arterial Interstate |
| 0.861 | 13.9 | | All | K,A,B,C | Principal Arterial Interstate |
| 0.918 | 8.2 | | All | O | Principal Arterial Interstate |
| 0.927 | 7.3 | | All | All | Principal Arterial Other Freeways and Expressways |
| 0.9 | 10 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways |
| 0.935 | 6.5 | | All | K,A,B,C | Principal Arterial Other Freeways and Expressways |
