Office of Operations
21st Century Operations Using 21st Century Technologies

Synthesis of Variable Speed Limit Signs



In most cases, variable speed limit (VSL) deployments are capable of generating desired traffic efficiency and safety system benefits. Because VSL systems have varying deployment goals and corresponding system design, varying system benefits resulted. Speed homogenization projects usually use simple algorithms in response to real-time traffic, road, and other conditions (e.g., weather, work zone, incidents, visibility, etc.), and report safety improvements. Multi-objective projects, mostly as a part of active traffic management (ATM) systems, report positive effects on mobility, safety, and even the environment. System benefits vary from site to site and it is difficult to generalize the reasons for these discrepancies due to many uncontrolled factors among different sites, such as compliance rate, driver behavior, or road geometry. However, VSL systems generally result in the following benefits:

  • Smoother traffic flow and less delay. As a component of ATM, VSL proactively manages speed to improve traffic flow and safety. Generally, some of the benefits of VSL include shortened queues, reduced congestion, quicker clearance during incidents, and fewer crashes. For example, Oregon has observed several of these benefits with a substantial reduction in speed differentials, improved harmonization, increased roadway capacity, and a reduction in crashes along OR 217.
  • Safer speeds in work zones. While agencies have found that performing nighttime construction reduces congestion and shortens traffic queues (compared to daytime construction), the lower volume also allows for faster speeds creating dangerous conditions in work zones. A VSL system allows the speed limit to be reduced so that vehicles approach construction areas and pass through work zones at safer speeds.
  • Ability to tie to road weather information system (RWIS) data to reduce speeds during inclement weather. When installing a VSL system for weather, many agencies can tie into existing RWIS stations to provide the data needed to determine when the speed limit should be reduced. Implementing VSL during adverse weather conditions can significantly improve safety and, in some cases, traffic efficiency.


Implementing a VSL system also comes with challenges, including enforcement of speeds that change, driver comprehension, and setting thresholds for speed limit changes (e.g., how much precipitation triggers a change, how often the signs should be updated, etc.). Although many VSL systems have been implemented across the country (and around the world), each site is unique and each system has its own characteristics and capabilities. Virginia DOT's challenges have included acquiring staff with vast capabilities, maintaining reliable system-wide communication, developing methods of encouraging compliance, and generating public approval through outreach activities (Earnest, 2015). Following are some of the other key challenges agencies experience when developing and implementing VSL systems.

  • VSL enforcement. Nearly every agency operating a regulatory VSL system reports challenges with speed enforcement. Law enforcement must know whenever the speed limit changes to be able to successfully enforce a VSL. In some instances, law enforcement may be hesitant to issue citations because they are unsure of the speed limit or fear a lack of supporting evidence for citations to be adjudicated.
  • Driver compliance. While enforcement of a regulatory system can be challenging, some agencies operating advisory VSL systems report a lack of driver compliance. Some agencies operate an advisory system due to current State statutes and agency policies. Others initiated their systems as regulatory, but changed to advisory after unsuccessful enforcement.
  • Hardware/software failures. Minnesota has experienced a shorter than expected life from their changeable speed limit signs while Nevada has seen their signs displaying different speeds when the VSL is activated and all displayed speed limits should be the same. Nevada has also had issues of their signs going blank and not displaying any speed limit. In addition, the New Jersey Turnpike Authority cited the constantly changing technology and necessary system upgrades as major challenges when maintaining a VSL system (United States Department of Transportation, 2002).
  • Lag in data. Depending on the source of the data or the algorithm used to analyze data, there can be a delay which results in the signage not displaying the appropriate speed limit for conditions. The New Jersey Turnpike Authority stresses the importance of calculating/posting the appropriate speed and ensuring that all variable messages are displayed/removed in a timely manner.
  • Returning the VSL back to the normal operating speed. One of the most challenging aspects operating a VSL is smoothly and efficiently returning the speed limit back to the regulatory speed limit following an issue along the roadway. The better the VSL system can transition drivers back to the regulatory speed limit, the better the system will be at preventing secondary crashes and keeping drivers safe in general.
  • Lack of cost/benefit information to support rationale for a VSL system. As highway agencies receive less funding, it is imperative to determine if a system's benefit will equal or outweigh the cost. There is limited information on cost/benefit analyses that agencies can use to support new implementation or expand existing systems.


This section provides a list of key factors that agencies should consider when implementing a VSL system. The report provides a more detailed discussion on the various items, but this list provides a summary for developing preliminary concepts for a VSL implementation.

General Considerations

  • First, develop some overall goals of what the VSL system should accomplish. It is important to note that VSL is not appropriate in all situations. Perform an analysis of whether or not VSL will be able to meet the overall goals.
  • The goals should include the desired situations in which speeds would be reduced (congestion, weather, work zone, etc.). The system design and further planning will depend on the overall situations in which the VSL system will be used.


  • The planning process should include a detailed systems engineering process to clearly identify and communicate objectives, requirements, and anticipated costs/benefits are crucial to successful implementations.
  • Based on VSL system goals and appropriate laws, carefully determine whether the system should be regulatory or advisory.


  • The infrastructure requirements will depend on the system purpose. VSL infrastructure requirements can include changeable Speed Limit Signs, weather/environmental sensors, traffic speed/volume sensors, and communications equipment to transmit data.
  • Selection of control algorithms also varies based on system goals. Algorithms can be difficult to calibrate so ample time should be spent on fine tuning the algorithms, particularly when incorporating real-time decisions based on congestion.
  • Systems can be set up to automatically display speeds or to provide recommendations for traffic management center staff for choosing to accept the recommendations. It is important to determine the method used for speed changes to occur.

Legal and Enforcement Considerations

  • Review State and local statutes and agency policies to ensure that a VSL system is enforceable if a regulatory speed limit is desired.
  • Begin meeting with law enforcement partners early in the process to discuss any concerns and processes for enforcing the VSL system, if enforcement is required.
  • Ensure that law enforcement personnel can safely enforce speed limits with potential safe places to stop violators, if enforcement is required.

Cost Considerations

  • When calculating the cost, be sure to account for items beyond the initial system cost, such as maintenance, operations, staffing, evaluations, and end-of-life replacement.


Following the comprehensive literature review and agency interviews, the research team has identified the following needs for future developments of VSL systems.

Variable Speed Limit Systems with Connected and Automated Vehicles

Highway technologies on information sharing and vehicle automation have made encouraging successes in recent years. Connected vehicle technology allows infrastructure units and vehicles to share high-resolution information from not only aggregated traffic, but also individual vehicles with other vehicles on the road, roadside infrastructure, and traffic management centers. With such connectivity, traffic operators can transmit traffic control information to individual drivers through wireless communication and in-vehicle devices. In addition to connectivity, connected and autonomous vehicle (CAV) technology enables vehicles to be controlled by precise, fast-responding, error-free computers instead of error-prone, slowly responding human beings.

CAV can be expected to provide much richer real-time traffic information (e.g., high-resolution vehicle trajectories) than traditional traffic sensors. Such information can be used to better understand what is happening, and what is to happen, with highway traffic, which is a new information basis for real-time traffic control. Studies have found that only a small market penetration percentage of CAV can yield very high benefits. Automation provides a new dimension for implementing traffic management strategies by directly regulating each individual vehicle's motion with precise, quickly responding computer algorithms. This will make it possible to extend traditional aggregated infrastructure-based traffic control to a disaggregated individual-vehicle-based control. This will achieve higher traffic efficiency, better safety and more comfortable individual driving (or riding) experience.

VSL speed-control algorithms should be updated to take advantage of these new technologies. Practically speaking, there will be a long period during which human drivers share the right-of-way with CAVs. In such mixed traffic scenarios, how to properly understand interactions between CAVs and manual vehicles, and how to utilize their interacting behavior and patterns to improve the system performance, is a highly relevant implementation issue yet to be addressed.

It is expected that with wide deployment of CAV technologies, traditional VSL systems that use gantries and variable message signs will gradually phase out. VSL or speed harmonization based on CAV technology, however, will require high market penetration. This is particularly critical for safety purposes and all drivers are supposed to be informed of hazardous and dangerous traffic and weather conditions. In the near term, VSL will still play an important role in traffic management systems and it will not be replaced completely by CAV technologies unless high market penetration, in some cases 100 percent for safety, is achieved and it is estimated that high market penetration of CAV technologies cannot be realized until 2030 (Underwood, 2016).

Collecting and Processing Big Data

Traditional VSL techniques only use aggregated traffic data obtained from regular point detectors (e.g., loop detectors, traffic cameras). Nowadays, increasingly advanced traffic sensors, such as in-vehicle Global Positioning System devices or connected vehicle technologies, provide higher resolution data with a wider coverage area: primarily, a more accurate aggregated traffic state (e.g., density, speed), and more detailed individual vehicle data (e.g., vehicle trajectories). The data generated from connected vehicle technologies (when fully deployed) will be much greater in quantity and much more complex in structure than traditional point detector data. How to utilize these data in the future VSL or connected-vehicle-based control paradigms is an interesting question. Real-time collection, storage, processing and decision-making using emerging big data sources is a promising VSL development.

Consideration of Driver Compliance

Driver compliance or driver response is a critical factor for effectiveness of VSL systems. Driver compliance rates, however, can vary dramatically across different projects due to information communication mechanisms, regulation, education, culture, and many other factors. Traditional VSL broadcasts uniform speed limit information via roadside infrastructure, and emerging connected vehicle technologies can send customized messages to each individual vehicle. Hayat et al. (2015) selected a small number of representative drivers to conduct a field test to evaluate driver compliance with different advisory messages, including VSL, lane change advisory, and merge control. From a human factors perspective, it is critical to understand how to design ATM signs. The recent FHWA ATM signage study (Perez et al., 2010) developed and tested alternative signs for variable speed limit (VSL) signs and used the deployments in Minnesota and Washington as inputs to sign development. Laboratory and field studies determined both the comprehension of the ATM signs as well as their respective legibility distances. Another major issue is how to make drivers believe that they would be better off (e.g., save time or reducing crash risks) if they comply with VSL messages and travel at a slower speed than they intuitively desire.

Further, VSL speed control algorithms should explicitly consider potential driver response or compliance rate. Considering driver compliance inevitably increases the complexity of such algorithms, but real-world deployments of these algorithms should consider the added complexity. Without pre-validated evidence of compliance rates, field studies should be conducted before implementing and tuning these algorithms in the real world.

Office of Operations