In 2014, acting on a ruling by the Texas Transportation Commission, a variable speed limit (VSL) pilot program was implemented by TxDOT and the Texas A&M Transportation Institute (TTI) was asked to study the effectiveness of temporarily lowering speed limits to address inclement weather, congestion, road construction, and other factors that can affect the safe and orderly movement of freeway traffic.
VSL systems implemented in the summer 2014 were evaluated at the following locations:
- Urban Congestion Site: San Antonio, WB SL604 from US 281 to IH-10. Data collected from June 30, 2014 – December 31, 2014
- Construction Work Zone Site: Temple, NB IH-35 NB from MM 297 to MM 301. Data collected from June 23, 2014 – November 30, 2014
- Weather-Related Site: Ranger Hill, Eastland County, EB and WB on IH-20 (proximal locations). Data collected from July 21, 2014 – January 30, 2015.
For the TxDOT pilot deployments, a 5-mi section of freeway upstream of a known bottleneck was used to demonstrate the concept. The section of freeway was divided into segments, generally between 0.5 and 1 mi in length. Each freeway segment represented a zone over which speed limits were varied approaching the bottleneck location. A variable speed limit sign was used to establish the recommended speed limit for travelers along the segment of freeway. Traffic sensors that measured speeds, volumes, and occupancies across all lanes were used to establish the speed limit to be displayed within the segment. The speeds were then displayed dynamically with VSL signs also associated with each segment.
In all cases, the VSL system issued an alert to the operator when a VSL needed to be initiated. The operator then had to confirm the alert to begin operating the signs in a VSL mode. Note that not all requests for activations were approved by operators. Once the signs were activated, the displayed speeds were updated every minute automatically without operator intervention.
Primary lessons learned from the pilot tests included the following:
- Use permanent equipment preferably mounted over travel lanes. Temporary equipment was not suitable for long-term operations.
- Fully understand existing speed profiles and ingress/egress characteristics on the corridor. Assure proper spacing for VSL systems and adjustments to sensor inputs.
- Plan for a large number of equipment and communications failures. Algorithms can be enhanced to address these issues and increase the public confidence in the messages posted by ensuring that they are correct and consistent. Test algorithm performance in shadow mode.
- Consider the need to share real-time data with other agencies, such as the Department of Public Safety.
- Conduct significant and on-going public outreach to assist drivers with understanding and complying with variable speed limits.
Based upon the data available for the VSL pilot program, VSLs would be beneficial if implemented to address inclement weather, congestion, or road construction.
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