A Variable Speed Limit (VSL) system on the I-270/I-255 loop around St. Louis reduced the crash rate by 4.5 to 8 percent, due to more homogenous traffic speed in congested areas and slower traffic speed upstream.

Evaluation of Variable Speed Limits on I-270/I-255 in St. Louis, Missouri

October 2010
St. Louis,Missouri,United States

Summary Information

The Missouri DOT (MoDOT) installed “Variable Speed Limit” (VSL) signs in May 2008 on the Missouri side of the I-270/I-255 loop around St. Louis. The objective of the VSL system was to prevent traffic flow breakdown in congested areas, thereby reducing delay and improving safety. Specifically, to encourage consistent speeds under congested conditions, an automated system set speed limits between 40 and 60 mph. Similarly, to reduce the closing speed of incoming traffic, the system reduced the speed limit upstream of congested areas. An evaluation estimated the effects of the VSL on changes in travel time, travel time reliability, and capacity, as well as safety and public and police perception of the system.


Post-VSL deployment data from four segments on I-270 during peak periods was used to evaluate mobility outcomes of VSL. To understand operations under virtual pre-VSL operations, the evaluation team analyzed “controlled” data, which was data collected during days without incident or adverse weather conditions. To estimate the effects of the VSL system on mobility, the team analyzed “uncontrolled” data collected on Tuesdays, Wednesdays, and Thursdays regardless of the occurrence of incidents and weather events were used.


The safety effects of the VSL system were estimated in terms of crash numbers and crash rates (number of crashes over traffic volume), and from two analysis methods (Naïve and Empirical Bayesian). Results consistently indicated that the VSL system reduced crashes from 4.5 to 8 percent with a standard deviation of 3.4 percent. The crash reduction is attributable to more homogenous traffic speed, reducing the change of rear-end or passing crashes, and slower traffic speed upstream of incidents, which may have reduced the potential for secondary crashes.

For Segment 1 (I-270 southbound approaching Manchester Road) and Segment 2 (I-270 northbound approaching I-44), the analysis of the “uncontrolled” data indicates that there was a 10% higher average traffic volume in post-VSL conditions, relative to pre-VSL conditions. The higher traffic volume in VSL conditions did not result in worse congestion. Comparisons between the “pre-VSL” and VSL conditions found a reduction in traffic congestion, demonstrating a mobility benefit from VSL. The analysis also suggests that increased driver compliance with the speed limits is likely to increase the benefits.

For Segment 3 (I-270 eastbound) and Segment 4 (I-270 westbound) both approaching I-270, results found that the VSL system had marginally beneficial effects in reducing the duration of peak periods and improving average speeds. However, the findings suggest that the system has potential for improvement by refining the initiation of the speed limit to each location’s anticipated flow rate, fixing faulty detectors, and increasing driver compliance with posted speed limits.

The VSL system had marginal improvements to overall mobility along the corridor, reducing the levels of congestion, shortening the duration of peak periods, and improving average speeds. However, the VSL system did provide safety benefits by reducing the number of crashes at a statistically significant level. The survey results found that the driving public and law enforcement expressed wide dissatisfaction with the VSL system in terms of congestion relief, compliance with the speed limit and visibility of the signs. The survey results suggest that stakeholders are unaware of the safety benefits of VSL, which provides an opportunity for further engagement on this topic by MoDOT.

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Evaluation of Variable Speed Limits on I-270/I-255 in St. Louis

Author: Bham, G.H., Long, S., Baik, H., Ryan, T., Gentry, L., Lall, K., Arezoumandi, M., Liu, D., Li, T., and Schaeffer, B.

Published By: Missouri Department of Transportation

Source Date: October 2010



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Related Metropolitan Integration Links

Link 13: Freeway Management to Incident Management

Typical Deployment Locations

Metropolitan Areas


VSL, managed lanes

Benefit ID: 2011-00735