Variable Speed Limits
Variable speed limits (VSLs) have long been used on European freeways to help manage speed and traffic flows in changing conditions (Katz et al., 2012). Variable or dynamic speed limits may be appropriate on roads with significant variations in congestion throughout the day or due to crashes, as well as roads with frequent weather conditions that affect safe travel speed. Sensors in the road detect congestion or weather conditions, then automatically lower the speed limit in stages to manage a more even slowing of traffic, delay of congestion onset, and smooth traffic flows. These changes plus lower speed have potential to reduce crashes, including secondary crashes. Infrastructure is needed to implement VSL active signs over all traffic lanes (generally), and digital communications such as variable message signs and monitoring infrastructure should also be in place (Katz et al., 2012). Use has been recommended for locations that experience variability in traffic volume and flow, weather and surface conditions that affect safe speeds, and other special situations such as work zones (National Center for Rural Road Safety, n.d.).
Lessons from States that have deployed VSL include challenges in traditionally enforcing variable speed limits since officers may not know what limit is in effect (Katz et al., 2012). States should ensure laws and procedures are in place so that charges will stand up in court. Other lessons include ensuring that the systems are only deployed in relation to need. Expert operators will need to periodically review and adjust algorithms that determine when the systems will be activated.
According to a synthesis report on variable speed limit signs for FHWA, at least 13 U.S. States provided information on VSL systems in use as of 2016. States that have used VSL systems to alter speed limits for weather conditions include Alabama, Delaware, and Washington (Katz et al., 2012). The systems have been used in several urban areas in various U.S. States and European countries to smooth traffic flows and reduce crashes during congested periods (Katz et al., 2012). Oregon has used VSL in both rural and urban areas (FHWA, 2020). More information is available from FHWA’s Active Transportation and Demand Management Deployments (Office of Operations, 2016). Variable speed limits or advisory limits have also been used in work zones in some States including Utah, Minnesota, and Texas.
VSL systems have been found to reduce crashes (especially rear-end crashes) and speeds in some cases, while improving flow capacity and travel time reliability in others (Katz et al., 2012). They have not worked well in all cases, however, and have been removed in some jurisdictions. A study of safety effects of variable limits deployed on freeways in the St. Louis area for congestion mitigation reported crash reductions of 8%. The congestion relief benefits were not as high as the public and agencies had hoped, however, leading to somewhat equivocal support for the measure (Bham et al., 2010). It appears that the VSL signs were posted only alongside the outer lanes, not over all lanes of the freeway.
An evaluation of a system deployed to address low visibility/heavy fog on I-77 in Virginia found significant reductions in mean speeds and speed variance during activation in the areas where fog was present, although not much change in speeds in transition areas leading to the fog areas (Gonzales & Fontaine, 2018). Preliminary investigation of a Wyoming freeway VSL system showed speed reductions from 0.47 to 0.75 mph for every mph reduction in speed limit (Buddemeyer et al., 2010). An assessment of average speed compliance to VSLs used on another Virginia freeway found that compliance was highest when lower VSLs were posted, but these may have related to higher congestion forcing drivers to slow (Boateng et al., 2019). Hard shoulders also seemed to encourage less compliance. A simulation study determined that implementing VSL upstream of merge areas, rather than downstream, would achieve greater safety benefits (Abdel-Aty & Wang, 2017).
While lower limits may be needed due to work zones, deploying VSL in these areas, in response to weather conditions or congestion, may help to ensure that lower speed limits are more appropriate to the conditions. Outcomes have varied, however, and stakeholders need to consult best practice guides to determine whether and how VSL may help to solve safety problems.
The cost will vary according to the type and extent of implementation. Capital costs have ranged from less than $50,000 to more than $5 million. A variable speed limit system covering 40 miles over the Snoqualmie Pass in Washington State was designed and implemented for $5 million in 1997. Operating costs for maintenance, energy, repairs, and potentially clearing the signs of snow/ice in winter should be included in estimates. See the Rural Intelligent Transportation Systems Toolkit (National Center for Rural Road Safety, n.d.), Variable Speed Limit (VSL) for more information.
Time to implement:
Determining the need, goals, and expected performance measures of VSL requires a comprehensive engineering and stakeholder review and will likely involve a medium to long time frame. Katz et al. (2012) provide an overview of need assessment, deployment, operations, maintenance, and regulatory and enforcement considerations for the use of VSL.