Use Case: PennDOT’s Implementation of Variable Speed Limit (VSL) Signs for Weather-Related Speed Management
Background
In 2020, a multi-vehicle pileup on I-80 involving more than 40 vehicles, including secondary crashes, resulted in one fatality and multiple injuries, underscoring the need for more advanced traffic management solutions in Pennsylvania. At the time, PennDOT was increasingly leveraging cameras and sensors from their Road Weather Information System (RWIS) to collect real-time road condition data. This enhanced "real-time awareness," coupled with the growing integration of Intelligent Transportation Systems (ITS), opened new possibilities for proactive traffic management.
As part of ongoing safety discussions with the Pennsylvania State Police, PennDOT explored the implementation of Digital Speed Limit (DSL) signs, also called Variable Speed Limit (VSL) signs, as an additional layer of speed management during severe weather events. These discussions, which focus on collaborative strategies for improving road safety, led to the adoption of VSLs as a reinforcement tool—complementing variable message signs (VMS) and state trooper presence rather than replacing existing measures.
Pilot Program and Early Success
PennDOT launched a pilot program in the 2021-2022 winter season in a single county known for severe winter storms and weather-related hazards such as fog. The selected corridor, which sits at the highest elevation east of the Mississippi, provided an ideal testbed for evaluating the effectiveness of DSLs in extreme conditions.
Speed and crash data were collected throughout the pilot. Results indicated a decline in crash frequency and a correlation between reduced corridor speeds and the activation of DSL signs. Given these positive outcomes, PennDOT deemed the pilot a success and began integrating the devices more broadly into its traffic management strategy.
Previous Speed Management Methods
Before adopting VSLs, PennDOT primarily relied on permanent and portable dynamic message boards controlled manually from the Central Traffic Management Center (CTMC). While effective, these signs often suffered from "awareness fatigue," as drivers grew accustomed to them and paid less attention to their messages—particularly when they remained static over long periods.
VSL signs provided a crucial improvement by introducing flashing beacons that activate when speed limits are lowered. This visual cue draws drivers’ attention more effectively than static signage. James Roman of PennDOT noted, " I find them to be a very effective tool, in addition to other tools like Changeable Message Signs (CMS) with automated stop/slow moving traffic messaging, to get people to slow down."
Impact and Effectiveness
Since their introduction, PennDOT has observed significant benefits from using VSLs, including:
• Crash Reduction: The ability to dynamically adjust speed limits has led to fewer crashes during severe weather conditions.
• Improved Speed Compliance: Drivers are more likely to adjust their speed based on VSL indications, particularly when reinforced by flashing beacons.
Initially deployed for winter weather conditions, PennDOT soon recognized the year-round value of VSLs. "They are working 24/7, 365 days," said Nicholas Minarchick of PennDOT District 2. The agency now leverages a robust network of RWIS sensors, cameras, CMS and field reports to determine when and where speed adjustments are necessary. Additionally, PennDOT coordinates with the State Police, who dispatches troopers as needed to reinforce speed compliance.
Over time, the agency has expanded the use of DSLs beyond winter storms to include:
• Weather-related hazards such as fog and heavy rain;
• Traffic management during roadway maintenance and striping operations;
• Incident response.
Today, VSLs are used at least a couple of times per week, demonstrating their growing role in PennDOT’s traffic management toolkit. "We are constantly finding creative ways to use them," Roman added. "Their versatility and efficiency make them a go-to solution for managing non-recurring congestion events."
Key Advantages of DSLs
PennDOT’s experience highlights two fundamental principles of effective speed management:
1. Relevance: By lowering speed limits only when and where necessary, DSLs encourage greater driver compliance.
2. Awareness: The flashing beacons used with DSLs ensure that drivers notice and respond to speed limit changes, unlike static signs that are often ignored.
Future Expansion and Lessons Learned
Encouraged by the program’s success, PennDOT is actively planning to expand VSL deployment, particularly in counties east and west of District 2, to create a more continuous corridor of dynamic speed management.
In terms of technology, PennDOT initially opted for trailer-mounted DSLs powered by solar arrays. However, early challenges—such as insufficient sunlight exposure in certain locations, particularly in late fall and winter— required the agency to adjust positioning of the units to enhance recharging efficiency. "We’ve had events where we needed to recharge units manually, especially along eastbound I-80, where sunlight was limited during these seasons," Roman explained. Minarchick also noted that they learned not to place units under trees to prevent shading issues.
Fortunately, PennDOT can remotely monitor battery levels for all units through cloud-based software that integrates with their entire DSL fleet. This capability allows the agency to proactively manage power levels and optimize deployment strategies ahead of weather events.
Conclusion
PennDOT’s implementation of DSLs has proven to be a highly effective strategy for managing speed during weather-related events and beyond. By leveraging real-time data and dynamic messaging, The agency has improved speed compliance, reduced crashes, and enhanced overall traffic safety. Their experience demonstrates that DSLs can serve as a flexible and efficient tool for both planned and emergency traffic management needs.
As PennDOT continues to expand its DSL network, other transportation agencies can look to this use case as a model for integrating dynamic speed management solutions into their own operations. The combination of real-time monitoring, strategic placement, and innovative technology integration ensures that DSLs will remain a valuable component of modern traffic management systems.