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

Synthesis of Variable Speed Limit Signs



The advancement of technological solutions in surveillance and control systems for traffic operations has led to significant reductions in the cost of implementing actively managed components of transportation systems. The benefit of these systems, particularly on highway-grade facilities, can be substantial. A few deployments have demonstrated the success of active traffic management (ATM) strategies. Recent studies conducted in Europe found that ATM applications have resulted in up to 22 percent of capacity increases and 30 percent of incident reductions. The key component in many of these actively managed systems is the variable speed limit (VSL) system. While the United States has installed VSL systems as far back as the 1960s on systems such as the New Jersey Turnpike, there has been a renewed interest in expanding their use in the United States during the past 15 years in order to achieve both operational benefits as well as proven safety benefits. The level and amount of positive impacts, however, vary from site to site, and there is a great interest in understanding the actual benefits under different operational scenarios.

Variable speed limits are typically installed on interstate highways or high-speed arterials and are used for three primary functions that can improve safety and operations: reducing congestion, reducing speeds during inclement weather, and managing speeds during traffic events such as work zones and incidents. Depending on State statutes and policies, speed limits can be either regulatory or advisory. Agencies use various data to inform the appropriate speed limit for current conditions. Using VSL, agencies can take into account a variety of conditions such as traffic volume, operating speeds, weather information, sight distance, and roadway surface conditions when posting speed limits. This data is typically transmitted to a transportation management center (TMC) and analyzed automatically with an algorithm or reviewed by agency personnel who make decisions about the speed limit. Some agencies use systems that will automatically change the speed limit based on the data received and others use data monitoring by personnel to change the speed limit manually. A majority of agencies use a hybrid approach with the VSL updating automatically supplemented with oversight by agency staff that have the option of overriding the automated system to manually change the speed limit when warranted.

VSL provides many benefits for improving roadway safety and operations. The use of VSL systems to manage speed during inclement weather or other challenging driving conditions can improve safety by decreasing the risks associated with traffic moving at speeds that are higher than appropriate for the conditions. In addition, VSL can dynamically manage speeds during planned (rush hour congestion) and unplanned (incidents) circumstances. Used in conjunction with managed lanes and other ATM strategies, VSL can help eliminate or delay bottlenecks and mitigate the possibility of rear-end, sideswipe, and other collisions generally associated with slowed traffic on high-speed roadways.


VSL has been successfully implemented in European countries for several decades, and deployments in countries such as the Netherlands and Germany have shown significant benefits. In Germany, VSL (i.e., speed harmonization) systems have been deployed since the 1960s, with installations on about 124 miles of highway. Germany's experiences show that VSL has the potential to decrease crash rates and increase road capacity by five to 10 percent. The Netherlands has implemented VSL since the 1970s in order to manage traffic speed, mitigate effects of extreme weather, and improve safety. The United Kingdom has implemented VSL and hard shoulder running on the M-42 motorway. An evaluation over a 12-month period showed the application of VSL and hard shoulder running resulted in seven percent increase in capacity, a 4 to 10 percent decrease in pollutants, and a 4 percent drop in fuel consumption.

In the United States, Michigan and New Jersey were the first two States to implement VSL. Speed limits at these two pioneer VSL sites were adjusted manually according to traffic conditions observed by traffic operations staff. Since 1990, VSL use in the United States has increased dramatically, with a renewed interest in expanding functionality to achieve operational benefits. VSL system complexity, in terms of infrastructure, signing, real-time detection and control algorithms, has significantly increased.

Over the years, agencies and researchers have published reports evaluating the effectiveness of VSL systems. Lu et al. (2014) and Ma et al. (2016) are two examples of comprehensive reviews of VSL speed control algorithms and resulting benefits from deployed systems. Generally, these systems have been proven effective in one or multiple performance measures on traffic efficiency, safety, and environmental impacts, depending on project characteristics such as deployment goals and speed control algorithms. These reviews, however, rely only on published reports and focus heavily on academic research. Many important issues such as VSL planning, policy, standards, design, and maintenance are not addressed systematically in these or other existing literature on VSL or ATM. This synthesis has been developed to provide a more comprehensive review of VSL by using agency interviews and additional internal agency documents to complement material from published reports.

Office of Operations