U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
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The purpose of any speed limit sign is to inform drivers of the maximum acceptable and safe speed for normal travel conditions. However, if roadway conditions are less than ideal, such as during wet weather conditions, conventional static speed limit signs may not display an appropriate, reasonable, and/or safe speed limit for those conditions. The use of variable speed limit (VSL) systems during inclement weather or other less than ideal conditions can improve safety by decreasing the risks associated with traveling at speeds that are higher than appropriate for the conditions. By using VSLs, agencies can take into account traffic volume, operating speeds, weather information, sight distance, and roadway surface condition when posting speed limits. An example of a VSL sign is shown in Figure 1. Wet weather conditions are a contributing factor to many crashes throughout the United States. Between 2000 and 2009, 12.6 percent of fatal crashes occurred on wet pavement (1). This chapter provides an introduction to VSLs and an overview of the important factors to consider when implementing a successful wet weather VSL system.
After the investigation of a wet weather fatal crash on Interstate 35 near Hewitt, Texas on February 14, 2003, the National Transportation Safety Board (NTSB) issued five safety recommendations to the Federal Highway Administration (FHWA) for improving traffic safety under wet weather conditions. These safety recommendations are as follows:
This project was conducted in response to recommendation H-05-14. The purpose of this project was to develop guidelines for using VSL systems in wet weather, particularly when operating speed exceeds design speed and stopping distance exceeds sight distance. The study team developed these guidelines after a thorough investigation of the state-of-the-practice of VSL installations in the United States and a complete review of the practices and procedures that States use when implementing and operating VSL systems. A table of the known VSL installations in the United States can be found in Appendix A.
This document also provides examples of the challenges, obstacles and issues that organizations have encountered when implementing VSL systems so that future implementers can develop practices that will increase their likelihood of success. These guidelines are intended for a broad range of audiences, from the transportation policy professionals who are considering whether or not their agency should use VSL systems to the engineers who are actually designing VSL systems for their jurisdictions. The information within this guidebook should be useful to anyone considering the implementation or development of a VSL system.
The American Association of State Highway and Transportation Officials (AASHTO) defines design speed as the speed used to determine various geometric design features of a roadway. The design speed is based on various geometric features but it does not account for all types of inclement weather. Operating speed is defined by AASHTO as the speed at which drivers operate their vehicles during free-flow conditions (2). Generally, the 85th percentile operating speed is used to calculate the posted speed of a facility. Although undesirable, in some cases the operating speed may exceed the design speed. The maximum safe speed is often independent of both the design speed on plans and the operating speed.
Although the term "maximum safe speed" is referred to in several different ways in literature, a universally accepted definition does not exist. In this report, maximum safe speed refers to the speed beyond which driving conditions would be deemed unsafe for a particular vehicle and thus could potentially result in a crash. These driving conditions refer to the existing roadway geometry, pavement surface condition, or weather conditions such as rain, snow, or fog. The reason that the maximum safe speed depends on the specific vehicle is each type of vehicle has different characteristics that can result in a different maximum safe speed. For example, a truck's maximum safe speed on a horizontal curve under wet conditions will most likely be lower than a car's maximum safe speed under the same conditions.
VSLs can be beneficial when the operating speed exceeds the design speed. This situation can occur at locations where there is a change in alignment or roadway surface condition and motorists travel through a curve at a speed that exceeds the design speed. Additionally, VSLs are beneficial when the operating speed is greater than the maximum safe speed. Since the maximum safe speed is based on conditions such as wet weather, a VSL can display a speed that matches the maximum safe speed for a particular condition. It is important to recognize that there are several factors that contribute to determining the maximum safe speed, including: design plans, roadway geometry, AASHTO assumptions, pavement friction, braking friction, and reaction times based on the best performing drivers and vehicles.
As appropriate design speeds and maximum safe speeds are determined for a given roadway segment, it is important that the relationship between sight distance and stopping distance is also considered. A driver must be able to see the roadway ahead and have adequate time to come to a complete stop when required. Calculations can be made to determine the available sight distance on both horizontal and vertical curves. Visibility sensors can be used to determine appropriate sight distance under rain, snow, or fog conditions.
The design speed determines the required stopping distance under design conditions. The maximum safe speed can be determined under wet or other adverse conditions using various coefficients of adhesion, available sight distance, and roadway grade. The coefficient of adhesion will vary based on the pavement surface characteristics (both micro- and macro-texture), vehicle operating parameters, tire properties, and environmental conditions. Stopping distance calculations take into account both the distance traveled during a driver's reaction time as well as the braking distance required for the vehicle to come to a complete stop. VSL systems can be used to reduce operating speeds so that the stopping distance is equal to or less than the available sight distance under less than ideal conditions, such as during wet weather. This topic is discussed in more detail in Chapter 2 of this Guidebook, while additional information on safe speed calculations can be found in Chapter 3.
Wet weather VSL systems should be considered in locations where there is a history of adverse weather conditions, where crash rates are higher than expected, where safe speeds are reduced more than 10 miles per hour (mph) below the posted speed limits, and where stopping distances exceed the available sight distance. The FHWA's ENTERPRISE Pooled Fund Study provided four warrants to consider when deploying regulatory VSL systems (3). While the ENTERPRISE Pooled Fund Study suggests that maximizing capacity and work zone safety are two reasons for considering VSLs, there are also warrants for safe stopping distance and safe travel speeds under adverse weather conditions. However, these warrants do not consider the situations when the stopping distance exceeds the sight distance. The warrants have been modified and simplified in Chapter 3.
There are many cases where static speed limit signs are effective in providing an appropriate driving speed for motorists. When wet weather conditions occur, drivers will typically adjust their speed in a manner that is safe for conditions. However, there are times, particularly when ideal conditions do not exist, when VSL systems should be considered in order to communicate appropriate speed limits based on real-time information. The most common implementations include locations with frequent adverse weather conditions, congestion, traffic incidents, and/or work zones.
Drivers typically have the ability to select safe speeds accurately under wet weather conditions on a straight roadway section with good pavement quality and adequate sight distances. However, if ideal conditions do not exist, and the roadway does not meet the driver's expectations, there is a greater chance that a driver error could result in a crash. For example, drivers typically cannot determine simply from driving on a pavement surface that their braking distances are increased due to poor available friction. Additionally, an unfamiliar motorist may not be aware of sight distance issues related to a horizontal curve until he or she enters the curve and has to apply the brakes suddenly. In situations such as these, VSLs may be an appropriate solution to communicate a safe driving speed to motorists. However, to be credible and effective, VSLs should be set for the average vehicle and driver and adjusted for actual weather and pavement conditions.
In considering when and where to deploy VSL as a weather-responsive traffic management strategy, agencies should also consider other countermeasures prior to installing a VSL system. These countermeasures can include installing "Slippery When Wet" signs, fluorescent yellow sign sheeting for enhanced conspicuity on signs, flashing beacons, and skid-resistant surface treatments, which could increase the critical speed to above the operating speed. In the vicinity of reduced sight distance due to horizontal curves, dynamic curve warning systems, horizontal signing, and optical speed bars may be considered. Further information on these and other countermeasures can be found in the following resources: