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Interstate 95 Variable Speed Limit System




I-95 is a vital conduit for north-south travel for the eastern seaboard for people, goods, and freight. The heavy traffic demand on this corridor, particularly during vacation and holiday seasons, contributes to frequent traffic congestion and crashes.

This area of I-95 is notorious for congestion caused by crashes and recurring delays during peak periods and weekends during vacation and holiday seasons. A 2017 nationwide study conducted by the Texas Transportation Institute ranked northbound I-95 between Exit 126 (US 1/Route 17) and Exit 143 (Route 610) as the seventh worst traffic hotspot in the nation with a projected cost to drivers of $1.1 billion.


Based on an analysis of crashes and the resulting delays in the corridor over a one-year period, a 15-mile section of northbound I-95 was selected for the implementation of a Variable Speed Limit (VSL) system with supporting traffic surveillance tools. This section of I-95 had limited TSMO strategies in place. With the known speed differentials and safety issues, the additional TSMO strategies of using VSL with CCTV and existing DMS were advanced. The greatest impact of this project will be the improved safety for travelers in this highly congested area of I-95.

TSMO Planning, Strategies and Deployment

VSL systems are used to improve safety and alleviate traffic. VSL signs replace traditional speed limit signs to reduce speed limits based on real-time traffic conditions. VDOT’s VSL system uses a custom algorithm based on data from traffic detectors to identify a speed limit (between 35 mph and 70 mph) that will best harmonize traffic flow. It anticipates the onset of congestion based on the detector data to proactively reduce speeds and maintain free-flow speeds. It also gradually slows traffic as it approaches congestion to reduce the risk of crashes.

VDOT assembled team to develop and approve a Concept of Operations. Lessons learned from other congestion-based VSL applications were incorporated into this system. A survey of existing non-weather-based systems showed most were primarily reactive to existing congestion, functioning mainly as queue warning systems. While valuable, in designing VDOT’s VSL system, significant attention was paid to developing a new algorithm that would be proactive in steadying traffic flow prior to breakdown, reacting quickly to delay the onset of congestion and reducing the adverse impact of bottlenecks when they do occur.

The team developed a three-step congestion-prediction algorithm that determines the current state of traffic, computes the speed recommendation for each site independently, and smooths the speeds to limit changes to speed limits at each sign and between adjacent signs. The result is a proactive system that incrementally reduces speed limits only when necessary to help ease congestion. This is the first VDOT VSL project to use per-vehicle record data by sensors that capture the speed of each vehicle. This data helps track compliance with posted speeds and enables the measurement of speed distributions.

Another unique aspect of this project is that the algorithm was developed using agile methods in Python by traffic engineers rather than software developers. Traffic engineers who understand traffic flow dynamics were able to iterate rapidly on algorithm updates and test continuously using recorded sensor data from the corridor and simulated conditions in VISSIM. All decisions made in the design of this VSL system were data-driven, from the selection of the pilot corridor to the creation of the congestion-prediction algorithm and the siting of the signs and sensors.

VDOT assembled a technical committee that was deeply engrained in the analyses for the initial design of the algorithm, the testing and validation of the algorithm during software development, and in the monitoring and performance review after the system went live. Prior to construction, VDOT deployed vehicle detectors on trailers to enable several months of algorithm testing, which enabled the smooth roll-out of the VSL system which included new CCTV. VDOT chose on-call contracting resources and completed construction in June 2022.

Communications Planning and Execution

Acknowledging that some travelers would drive along this section of I-95 every day during a work commute, while for others, their exposure might be once a year on vacation, the project’s public awareness campaign was designed to help all travelers understand how to navigate the project corridor safely.

The tagline, “Reduce your speed when there’s a need,” emphasized that drivers should slow down when required due to congestion or an incident ahead. In addition, a video was created showing rice poured through a funnel to demonstrate the concept of speed harmonization. When the rice was poured rapidly into the funnel, the rice clogged the funnel’s opening. But it kept moving when the rice was poured into the funnel at a slower but consistent rate. This video was especially effective during media interviews and was even used by a reporter in the broadcast coverage.

Additional outreach methods included posting physical signs at interstate Safety Rest Areas closest to the VSL zone to reach drivers who may infrequently pass through Virginia.

Mobile digital advertising reached drivers approaching the zone, and “zero speed” advertising on Waze reached travelers in the app who stopped nearby for food or gas. In addition, a billboard was posted on I-95 northbound just before the zone. Finally, VDOT’s social media channels included messaging about the launch, and information materials such as a project video and Frequently Asked Questions were available on the project website.

A media event with project leadership was held at a Safety Rest Area targeting out-of-state travelers near the project’s activation site before the launch to answer questions about the project’s purpose and explain how it worked and how drivers would navigate the corridor.

During design, VDOT held briefings with the MPOs and law enforcement communities.

Outcome, Learnings and Public Benefit

VDOT’s Variable Speed Limit (VSL) system is a cost-saving, productivity-enhancing, and service-enhancing solution. The VSL system provides service enhancements by reducing crashes and stop-and-go conditions. Early evaluations of the system demonstrate an increase in driver awareness of downstream congestion leading to safety benefits. The VSL system provides additional service enhancements by reducing speed differentials between lanes and reducing delays caused by congestion.

The VSL system is proactive in steadying traffic flow prior to breakdown and reacting quickly to delay the onset of congestion. As a result, Northbound I-95 travelers are benefiting from fewer severe crashes on this 15-mile section of interstate, based on initial data since the system was activated in June 2022. And while drivers may still need to reduce their speed in this area due to congestion, they are encountering fewer sudden halts with less hard braking as the VSL signs gradually step travelers down to a safe, stable speed.

Data from June 22, 2022, to February 20, 2023, showed reduced crash rates for all types of crashes compared with the same period a year earlier before activation. There was a 22% reduction in fatal and injury crash rates and a 9% reduction in rear-end crash rates. While this is early data, it is promising for improvements to traveler safety. Data from this same period also showed a marked decrease in speeding behavior when the speed limit is posted at a reduced limit of 55 mph based on comparable conditions in the prior year. In addition, the variance of speeds between vehicles and across lanes is reduced, signifying a change in driver behavior.

Organizational Capability Element

    Freeway Operations
    Freight Management Operations

Content Type

Case Studies & Lessons Learned

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