Guidelines for the Use of

Variable Speed Limit Systems in Wet Weather

FHWA Safety Program

FHWA-SA-12-022 http://safety.fhwa.dot.gov

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FHWA-SA-12-022

FHWA Project Manager – Guan Xu

FHWA Technical Reviewers – Richard Knoblauch, Ed Rice, Roemer Alfelor

This report provides guidance on the use of variable speed limit (VSL) systems in wet weather at locations 

where the operating speed exceeds the design speed and the stopping distance exceeds the available 

sight distance. The use of VSLs 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. 

This report provides guidelines for the design, installation, operation, maintenance, and enforcement of wet 

weather VSL systems. The guidelines presented in this report 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. 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.

Federal Highway Administration

U.S. Department of Transportation

Office of Safety 

1200 New Jersey Avenue, SE 

Washington, DC 20590

Variable Speed Limits, Wet Weather, Roadway 

Conditions, Pavement Conditions, Visibility, Sight 

Distance, Traffic Management Center

No restrictions. This document is available to 

the public from: The National Technical Information Service, Springfield, VA 22161. 

Unclassified Unclassified 40 N/A

2. Government Accession No. 3. Recipient’s Catalog No.

August 24, 2012

Bryan Katz, Cara O’Donnell, Kelly Donoughe, 

Jennifer Atkinson (SAIC)

Melisa Finley, Kevin Balke , Beverly Kuhn (TTI)

Davey Warren (Brudis and Associates)

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

DTFH61-10-D-00024

Science Applications International Corporation

8301 Greensboro Drive

McLean, VA 22101
iv | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather
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Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

Table of Contents

Chapter 1. Introduction......................................................................................................................................... 1

1.1 Background ................................................................................................................................................ 1

1.2 Roadmap to the Guideline Document..................................................................................................... 2

1.3 Design Speed, Operating Speed, and Maximum Safe Speed .............................................................. 2

1.4 Sight Distance and Factors Affecting Stopping Distance....................................................................... 3

1.5 When to Consider Variable Speed Limit Systems .................................................................................... 3

1.6 Other Countermeasures to Consider ....................................................................................................... 4

Chapter 2. Driver, Vehicle, and Roadway Characteristics Related to Driving in Wet Weather.................. 4

2.1 The Driving Task .......................................................................................................................................... 4

2.2 Sight Distance............................................................................................................................................. 5

2.3 The Driver..................................................................................................................................................... 6

2.4 The Vehicle ................................................................................................................................................. 6

2.5 The Visual Scene ........................................................................................................................................ 7

2.6 The Pavement............................................................................................................................................. 7

2.7 Speed Zoning ............................................................................................................................................. 8

Chapter 3. Guidelines for the Design of Wet Weather Variable Speed Limit Systems ................................. 9

3.1 Determining the Appropriate Type of Variable Speed Limit System to Use ........................................... 9

Guideline 1: Conduct an analysis to determine if a wet weather variable speed limit 

system is appropriate........................................................................................................................................ 9

Guideline 2: A regulatory variable speed limit system is preferable over an advisory 

variable speed limit system............................................................................................................................ 10

Guideline 3: Consider a semi-automated or automated approach for variable

speed limit systems ......................................................................................................................................... 12

3.2 Determining Speed Limits for Variable Speed Limit Systems................................................................. 12

Guideline 4: Incorporate weather responsive decision support into existing variable 

speed limit algorithms to determine the displayed speed limit.................................................................. 12

Guideline 5: All speed limit algorithms and manual display calculations should be 

approved by a traffic engineering professional ........................................................................................... 15

Guideline 6: For freeways, set a minimum speed limit of not less than 30 mph for 

regulatory systems or not less than 15 mph for advisory speed systems.................................................... 15

Guideline 7: Use speed limits in 5 mph increments...................................................................................... 15

3.3 Determining Display and Location of Variable Speed Limit Signs ....................................................... 15
vi | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

Guideline 8: Display variable speed limit changes for at least 1 minute................................................... 15

Guideline 9: Do not display reduced speed limits more than 1 mile upstream from

the section of roadway where the reduced speed is desired..................................................................... 16

Guideline 10: Where variable speed limit signs are closely spaced, do not allow 

speed differentials of more than 15 mph without advance warning ......................................................... 16

Guideline 11: Use Changeable Message Signs (CMS) to communicate reasons

for speed reduction ........................................................................................................................................ 16

Guideline 12: As with static speed limit signs, variable speed limit signs should be

placed at all entrances to the roadway ........................................................................................................17

Chapter 4. Guidelines for the Design, Installation, Operation, and Maintenance

of Wet Weather Variable Speed Limit Systems .................................................................................................17

4.1 Guideline 13: Develop a comprehensive concept of operations for the variable 

speed limit system............................................................................................................................................17

4.2 Guideline 14: Install appropriate weather sensors or use accurate weather data 

at problem locations .......................................................................................................................................17

4.3 Guideline 15: Ensure that equipment required in the design process is 

incorporated into the installation requirements ........................................................................................... 18

4.4 Guideline 16: Develop an Operations and Maintenance Plan............................................................ 18

Chapter 5. Guidelines for the Enforcement of Wet Weather Variable Speed Limit Systems ..................... 19

5.1 Guideline 17: Coordinate and collaborate with law enforcement before

deploying a variable speed limit system ...................................................................................................... 19

5.2 Guideline 18: Determine the legal authority to post variable speed limits .......................................... 20

5.3 Guideline 19: Maintain system log for evidence of speed limit violations ........................................... 20

Chapter 6. Weather-Related Variable Speed Limit Case Studies................................................................. 20

6.1 Alabama................................................................................................................................................... 21

6.2 Delaware................................................................................................................................................... 21

6.3 South Carolina.......................................................................................................................................... 22

6.4 Washington............................................................................................................................................... 22

6.5 Wyoming ................................................................................................................................................... 24

6.6 Discussion ................................................................................................................................................. 24

Appendix A. References..................................................................................................................................... 26

Appendix B. VSL Installations in the United States.......................................................................................... 28
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Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

List of Tables

Table 1. Alabama DOT Low Visibility Warning System Strategies ...................................................................... 21

Table 2. Washington State DOT Speed Management Control Strategies ........................................................ 23

Table 3. Wyoming DOT Speed Management Strategies ................................................................................... 24

List of Figures

Figure 1. VSL Sign Installation in Pennsylvania...................................................................................................... 1

Figure 2. Wet Weather Variable Speed Limit Flowchart ..................................................................................... 10

Figure 3. Regulatory Versus Advisory VSL Flowchart............................................................................................11

Figure 4. Example Regulatory and Advisory Variable Speed Limit Signs ..........................................................11

Figure 5. Example VSL Algorithm for Wet Weather ............................................................................................. 14

Figure 6. Variable Speed Limit System on the New Jersey Turnpike.................................................................. 16

Figure 7. Changeable Message Sign in Wyoming Used to Communicate 

Speed Reduction Warning....................................................................................................................................17
viii | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather
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Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

Chapter 1.Introduction

1.1 Background

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:

1. H-05-12 – Issue guidance to your field offices describing the inadequate stopping sight distance 

that could occur when poor vertical geometries exist at locations with low coefficients of friction 

and speeds higher than the design speed, and work with States to inventory such locations. 

2. H-05-13 – Once the locations in Safety Recommendations H-05-12 have been identified, assist the 

States in developing and implementing a plan for repaving or other roadway improvements. 

3. H-05-14 – Issue guidance recommending the use of VSL signs in wet weather at locations where 

the operating speed exceeds the design speed and the stopping distance exceeds the available 

sight distance. 

4. H-05-15 – Conduct research on commercial vehicle tire and wet pavement surface interaction 

to determine minimum frictional quality standards for commercial tires on wet pavement; once 

completed, 1) revise the tire requirements for commercial vehicles operating on wet pavement 

at highway speeds, and 2) develop minimum acceptable pavement coefficients of friction and 

maximum permissible pavement rut depths as part of roadway maintenance requirements, as 

appropriate. 

Figure 1: VSL Sign Installation in Pennsylvania

(Source: Bryan Katz)
2 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

5. H-05-16 – Review State programs that identify and eliminate locations with a high risk of wet 

weather accidents and develop and issue a best practice guide on wet weather accident 

reduction. 

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.

1.2 Roadmap to the Guideline Document

•	 Chapter 1 (this chapter) provides an introduction to VSL and the guidebook itself. 

•	 Chapter 2 identifies driver, vehicle, and roadway characteristics affected by wet weather 

driving conditions. 

•	 Chapter 3 provides design guidelines on setting appropriate speed limits and other 

recommendations related to the signage and development of algorithms to use for VSL 

systems in wet weather. 

•	 Chapter 4 provides guidelines on the installation, operation, and maintenance of VSL systems 

in wet weather. 

•	 Chapter 5 provides guidelines on coordinating VSL system installation with law enforcement 

and the judicial system. 

•	 Chapter 6 describes case studies of agencies that have implemented successful VSL systems 

for wet weather conditions. 

1.3 Design Speed, Operating Speed, and Maximum Safe Speed

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 freeflow 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 
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Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

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.

1.4 Sight Distance and Factors Affecting Stopping Distance

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.

1.5 When to Consider Variable Speed Limit Systems

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 
4 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

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.

1.6 Other Countermeasures to Consider 

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:

•	 Engineering Countermeasures for Reducing Speeds. U.S. Department of Transportation, Federal 

Highway Administration, Washington, D.C. Available at http://safety.fhwa.dot.gov/speedmgt/ref_

mats/eng_count/. Accessed on April 17, 2012.

•	 McGee, H.W. and F.R. Hanscom. Low-Cost Treatments for Horizontal Curve Safety. FHWA-SA-07-002. 

U.S. Department of Transportation, Federal Highway Administration, Washington, D.C., December 

2006. Available at http://safety.fhwa.dot.gov/roadway_dept/horicurves/fhwasa07002/. Accessed 

on July 3, 2012.

Chapter 2. Driver, Vehicle, and Roadway Characteristics Related to Driving in 

Wet Weather

2.1 The Driving Task 

 A driver is continuously balancing three main tasks: control, guidance, and navigation. Control refers to 

all activities involved in the driver’s interaction with the vehicle. The driver receives control information from 
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Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

the vehicle’s displays, from the “feel” of the vehicle, and from the roadway. Control also involves the driver 

maintaining a safe speed and keeping the vehicle on the proper path (4, 5).

Guidance of the vehicle is how the driver interacts with other vehicles, the roadway, and the surrounding 

environment to guide the vehicle safely down the roadway. As drivers travel along the roadway, they 

receive information (e.g., alignment, grade, hazards, traffic, traffic control devices, etc.) from various 

sources and use their judgment to adjust the vehicle’s speed and path appropriately (4). 

Navigation is how the driver is going to get from their origin to their destination. Navigation involves both 

pre-trip and in-trip decisions. Pre-trip navigation can include planning a route, receiving instructions, 

mapping a route, etc. In-trip navigation includes using route guidance and landmarks en-route to 

get from the origin to the destination (4). Drivers may also use electronic navigation devices or global 

positioning system (GPS) units to obtain navigation information en-route.

At any given time, a driver is receiving and processing information that contributes to decisions being 

made regarding one or more of these tasks. Drivers continuously use information from a number of realtime sources as well as their experiences and general expectations to make driving decisions. Drivers 

“juggle” these multiple sources of information by giving any one piece of information a short amount 

of attention before moving onto the next piece of information. Drivers should prioritize the relative 

importance of the information being received. Importance is assigned based on the consequence of not 

paying attention to an information source. Information with the highest importance should be addressed 

first. Obviously, among the main driving tasks, control is the most important, followed by guidance 

and navigation. Thus, under less than ideal conditions, such as wet weather, drivers tend to pay more 

attention to the control of the vehicle and less attention to less critical information (4).

2.2 Sight Distance

For the safe and efficient operation of vehicles, drivers need to be able to see the roadway ahead and 

any potential hazard soon enough to react to it. Sight distance is the length of roadway ahead that is 

visible to a driver. Sight distance is comprised of driver perception-reaction time and the time to complete 

the maneuver safely. The driver perception-reaction time includes the time to perceive the condition 

(e.g., an object in the roadway), time to complete cognitive functions to understand the condition (e.g., 

recognize the object and decide if and how to respond), and time to initiate the desired maneuver 

(e.g., take foot off accelerator and step on brake). The driver perception-reaction time varies based on 

the distance to and nature of the object, the visual performance of the driver, the cognitive capabilities 

of the driver, the atmospheric visibility, and the roadway environment. The time to safely complete the 

maneuver is primarily impacted by the roadway condition and the vehicle’s performance capabilities, 

especially tire-pavement friction, road-surface conditions (e.g., wet), and downgrades (6).

In general, sight distance should be sufficient to allow a driver traveling at or near the design speed 

to stop before reaching a stationary object in the travel path. Stopping sight distance is the distance 

traveled from the instant the driver sees and recognizes a hazard to the instant the brakes are applied 

(brake reaction time) plus the distance actually needed to stop the vehicle (braking distance). A 

Policy on Geometric Design of Highways and Streets (2) provides a thorough explanation of stopping 

sight distance as well as the design values and stopping sight distances for various design speeds 

based on assumed conditions on level roadways. The design values are determined using values for a 

predetermined design driver and design vehicle under a predetermined set of circumstances. However, 
6 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

the stopping distance can vary based on factors, including (but not limited to) driver reaction times, 

vehicle deceleration rates (which are different for each type of vehicle), braking efficiency, coefficient 

of adhesion, and roadway grade. The coefficient of adhesion refers to the frictional force between the 

tires and the pavement. Thus, the coefficient of adhesion is lower under poor tire conditions and poor 

pavement conditions than it would be for good tire conditions on dry pavement. A decrease in the 

coefficient of adhesion will result in an increase in stopping distance. Using longer distances increases 

the margin for error for all drivers, especially during wet weather conditions. However, as discussed in 

Chapter 1, there are roadway environments where the operating speed is greater than the design speed 

and the safe stopping distance is greater than the available sight distance. 

2.3 The Driver

In 2010, there were more than 210 million drivers in the United States. Approximately half of these drivers 

were male and half were female. Almost three-quarters of these drivers (71 percent) fall into the 25 to 64 

age range, with younger drivers (24 or less) comprising 13 percent of the driving population and older 

drivers (at least 65) representing 16 percent of the driving population (7). However, it is expected that the 

older driving population will double over the next 30 years (8). The changing demographics of the driving 

population are important to recognize when considering the impacts of wet weather on the driving task. 

Typically, drivers with more experience are less risk tolerant than younger drivers. Driver experience and 

maturity impacts decision-making, especially under less than ideal conditions, such as wet weather. For 

example, in wet weather conditions, younger drivers may tend to devote less attention to control of the 

vehicle than they should. Brake reaction times can vary between drivers from as low as 0.64 seconds for 

alerted drivers to over 3.5 seconds for un-alerted drivers when simple signals are used in research tests (2).

2.4 The Vehicle

In 2010, an estimated 242 million motor vehicles were registered in the United States (7). These motor 

vehicles consist of passenger cars, light trucks and vans, motorcycles, recreational vehicles, buses, 

and commercial vehicles, all of which have different design and operational characteristics. Thus, the 

amount of control needed to drive a vehicle safely varies based on the type of vehicle. For example, it 

is less demanding to control a passenger vehicle with an automatic transmission than it is to control a 

commercial vehicle with multiple gears and clutches. Even so, most control activities, once mastered, 

are performed automatically with little conscious thought (4). A vehicle’s design and maintenance also 

impact a driver’s ability to control the vehicle, especially during wet weather. Anti-lock brakes reduce 

wheel lock-up under braking, which reduces the likelihood of skidding on slippery pavement. Vehicle 

maintenance items include, but are not limited to, properly working windshield wipers, proper tire 

pressures, adequate amount of tread on tires, and maintenance of the brake system. AASHTO suggests 

that although most drivers decelerate at rates greater than 14.8 ft/s2 when an unexpected object is 

encountered in the roadway, 90% of drivers decelerate at rates greater than 11.2 ft/s2, and thus 11.2 ft/s2 is 

used as a design value (2). 

The previously mentioned stopping sight distances are typically based on passenger cars, so they do 

not explicitly consider commercial vehicles. In general, commercial vehicles need longer stopping sight 

distances than passenger cars. However, because commercial vehicle drivers are positioned higher 

above the pavement, they can generally see further down the road than a driver of a passenger car. 

Thus, commercial vehicle drivers typically have more time to perceive and react to a potential hazard, 
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Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

somewhat making up for the increased braking distances. So, separate stopping sight distances for 

different vehicle types are not typically used in highway design. One exception includes locations 

where horizontal sight restrictions exist on downgrades, especially at the end of long downgrades where 

commercial vehicle speeds are similar to or exceed passenger car speeds. Under such conditions, it is 

desirable to provide stopping sight distances longer than those provided for passenger cars (2). Another 

situation where commercial vehicle drivers may not be able to see further than passenger car drivers 

is during rain. Under these conditions, the intensity of the rain may limit the view of the roadway and 

surrounding environment of all drivers.

2.5 The Visual Scene

Wet weather reduces the visibility of potential hazards and traffic control devices, making them more difficult 

for drivers to detect. The distance at which a driver can see a non-illuminated, non-reflective object is 

dependent upon a variety of factors, including the vehicle’s headlights, the driver’s visual capabilities, and 

the driver’s expectation of seeing a hazard. While many traffic control devices are retroreflective, rain can 

reduce the retroreflective materials’ ability to perform and decrease the visibility of the device. Under wet 

weather conditions, internally illuminated traffic control devices may provide better visibility.

As discussed, under normal driving situations drivers have the ability to process information from multiple 

sources, but as conditions deteriorate, they begin shedding information sources in order to attend to the 

primary driving functions of control and guidance. Traffic control devices (e.g., speed limit signs) have to 

compete for the driver’s attention; therefore, it is critical that they be placed where drivers can see them 

and quickly obtain information. 

2.6 The Pavement

As mentioned, the time to safely stop once a hazard is detected and the driver applies the brakes is 

dependent upon road surface conditions and available tire-pavement friction. Pavement conditions that 

may contribute to an increase in wet weather crashes include (9):

•	 Rutting or wheel path channelization.

•	 Build-up on shoulder edges that causes ponding on the road surface.

•	 Bleeding of pavements.

•	 Drainage issues that result in water on the pavement.

•	 Geometrics (grades and curvature).

Water on the pavement can significantly reduce tire-pavement friction. Even as little as 0.002 inches of 

water on the pavement can reduce the coefficient of friction by 20 to 30 percent (10). 

Rutting, build-up on the shoulder, or drainage issues can result in relatively thick water films on the 

pavement surface. The coefficient of friction for a vehicle tire sliding on wet pavement decreases 

exponentially as the water film thickness increases. The effect of the water film thickness is influenced by 

tire design and condition, with worn, smooth, and “bald” tires being most sensitive. At low speeds (less 

than 20 mph), the effect of water film thickness on friction is minimal; however, at higher speeds (greater 

than 40 mph), the effect is more pronounced. When relatively thick water films are present and vehicles 

are traveling at higher speeds, hydroplaning can occur (9).
8 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

Obviously, the deceleration maneuver is impacted by wet pavement. A Policy on Geometric Design 

of Highways and Streets (2) recommends 11.2 ft/s2, a comfortable deceleration rate for most drivers, 

as the deceleration threshold for determining adequate stopping sight distance. Such deceleration 

corresponds to the braking friction on packed snow and thus is within the driver’s capability to maintain 

control of the vehicle during braking maneuvers on wet pavement. Also, most vehicle braking systems 

and the tire-pavement friction levels on most roadways, even under wet conditions, are capable of 

providing a deceleration rate of at least 11.2 ft/s2.

Previous research has shown that under emergency conditions, mean deceleration rates are the same 

between good and poor visibility conditions, but differ among good and poor traction conditions (17.7 

ft/s2 and 13.8 ft/s2, respectively). However, the 85th percentile values are the same (12.1 ft/s2). On wet 

pavement with vehicles without an anti-lock braking system, the mean and 85th percentile deceleration 

rates are about 13.8 ft/s2 and 12.2 ft/s2, respectively (54% and 47% of the pavement’s coefficient of friction, 

respectively). On wet pavement with anti-lock brakes, the mean and 85th percentile deceleration rates 

are about 17.1 ft/s2 and 14.5 ft/s2, respectively (66% and 56% of the pavement’s coefficient of friction, 

respectively) (6). 

2.7 Speed Zoning

All States follow the same basic approach to formulate speed regulations. This approach specifies that 

drivers should not exceed speeds that are safe and prudent for existing conditions, even if the posted 

speed limit indicates that a higher speed is allowable. Thus, a driver is responsible for considering the 

roadway environment and potential hazards when selecting a speed. This includes weather, visibility, 

traffic, and roadway characteristics (5). 

A safe and reasonable speed limit is typically based on a statutory speed limit or traffic engineering study. 

Speed zones established based on these studies may change the basic speed limits set by law. Speed 

zones can either be regulatory or advisory. Regulatory speed limits are enforceable, whereas advisory 

speed limits are generally not enforceable, although drivers could be cited for violating the basic speed 

rule that requires drivers to drive at an appropriate reduced speed when necessary (5).

In most cases, the establishment of a speed zone is predicated on the assumption that most drivers 

operate their vehicles in a safe, reasonable, and prudent manner. The speeds that the majority of drivers 

choose to travel on a given roadway segment are considered an indication of safe and reasonable 

speeds. The 85th percentile speed of free flowing vehicles, the speed at which 85% of drivers travel at 

or below at a given point on the roadway, is commonly taken to determine the posted speed for that 

segment. This driver-defined maximum safe speed can then be adjusted slightly, if necessary, based on 

site-specific factors, including sight distance restrictions, pavement conditions, etc. A speed limit to the 

nearest 5 mph increment of that maximum safe speed is then typically posted (5, 11). 

Even though the speed limit may be based on the 85th percentile speed, many studies have reported that 

the posted speed limit is usually significantly lower than the measured 85th percentile value. For example, 

the Institute of Transportation Engineers (ITE) found that for roadways with posted speed limits of 45 mph 

and below, most of the measured speeds are higher than the posted speed limit (12). When the posted 

speed limit is 55 mph or more, about half of the measured speeds are above the posted speed limit. This 

may be due in part to the difficulty in predicting operating speeds (and thus the speed limit) based on 

the roadway geometry and roadside features.
| 9

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

As discussed previously, all basic State speed laws typically require drivers to adjust their speed to existing 

road conditions; thus, the primary responsibility for adjusting speed to adverse conditions, such as wet 

weather, rests with the driver. Nevertheless, in an attempt to improve safety, some jurisdictions have 

decided to reduce speed limits at specific locations during adverse weather conditions. 

A VSL is one that changes with changing conditions. The use of this operational strategy is by no means 

new; dating back more than 60 years, agencies have modified speed limits on a temporary basis for a 

variety of reasons. School zones and work zones are some of the more common applications of VSLs. 

Modern VSL systems use technology to adjust speed limits in real-time. Speeds may be modified based 

on traffic conditions, adverse weather conditions, road surface conditions, or work zones to better control 

operating speeds within the monitored section of roadway. VSLs not only work to maintain safe driving 

speeds, but also to warn drivers of changing road conditions. However, when wet weather conditions 

get too bad, the speed limit displayed on the VSL could still be “faster” than what drivers feel is a speed 

at which they can comfortably control and guide their vehicle. This can be problematic because drivers 

trust VSL systems to provide them with the maximum safe speed for the conditions.

As noted, driver behavior varies during adverse weather events based on experience and risk tolerance. 

Therefore, some drivers may slow down when they encounter wet weather, but others may maintain or 

even increase their speeds. The result of this varying behavior may be a significant speed differential 

between slower and faster vehicles on the roadway. It is generally accepted that an increase in speed 

differential increases the potential for conflicts between vehicles, which in turn increases the potential 

for crashes. VSL systems based on weather conditions can work to reduce the speed differential under 

unpredictable events and encourage more uniform speeds. However, there is still the potential for speed 

differentials to occur immediately upstream of the rain event and reduced speed limit (i.e., as drivers enter 

the reduced speed limit zone). 

Chapter 3. Guidelines for the Design of Wet Weather Variable Speed Limit 

Systems

This chapter provides design guidelines on setting appropriate speeds and other details related to speed 

limit signage and the development of algorithms for the use of VSL systems in wet weather. This chapter 

includes guidelines for determining the appropriate type of VSL system to use, setting appropriate speed 

limits for VSLs, and determining the display and locations of VSL signs.

3.1 Determining the Appropriate Type of Variable Speed Limit System to Use

Guideline 1: Conduct an analysis to determine if a wet weather variable speed limit system 

is appropriate.

Figure 2 depicts a decision-making process for assessing whether a Wet Weather VSL is appropriate for 

a given location. If a location is not appropriate for a VSL system, then other countermeasures may be 

appropriate (refer to section 1.6).

Whether or not a Wet Weather VSL system should be considered is dependent on an area experiencing 

adverse weather conditions that impact traffic, a higher crash rate than expected for similar segments, 

a regularly occurring speed requirement that reduces operating speeds 10 mph less than the typical 

posted speed limit, and conditions where stopping distance for average drivers and vehicles exceeds the 
10 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

NO

YES

YES

YES

YES

NO

NO

NO

NO

Wet weather

VSL system

is justified.

Does the area experience

adverse weather conditions 

with traffic problems, slow

downs, or safety hazards? 

Is the crash rate higher

than expected for similar

segments?

Is there a regularly occuring

safe speed requirement of at

least 10 mph less than

normal posted speed?

Are there conditions

in which stopping

distance exceeds the

available sight distance?

Typically does not justify

wet weather VSL system;

consider other

countermeasures.

May consider wet

weather VSL system.

available sight distance, based on the physical condition of the road. These conditions might exist due to 

alignment changes or weather conditions such as snow, rain, or fog.

When all guidelines are met, a Wet Weather VSL installation would be justified. If the rest of the guidelines 

are met and there are no conditions where the stopping distance exceeds the available sight distance, a 

Wet Weather VSL may still be considered, based upon traffic engineering judgment. 

Guideline 2: A regulatory variable speed limit system is preferable over an advisory variable speed

limit system.

It is important to recognize that a regulatory system can be enforced, while an advisory system typically 

cannot. A regulatory system is preferable, especially for longer segments, because it usually results in a 

much higher compliance rate. However, it also requires a larger responsibility to display accurate realtime data and to be credible to most drivers. This is particularly important when using VSLs in wet weather, 

Figure 2: Wet Weather Variable Speed Limit Flowchart
| 11

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

because a regulatory system displaying speeds that are not based on real-time weather conditions 

can result in inappropriate speed recommendations. Figure 3 illustrates a decision tool for determining 

whether a system should be regulatory or advisory. The majority of VSL systems currently active in the 

United States are regulatory and therefore enforceable. Examples of regulatory and advisory VSL signs 

are shown in Figure 4. The main consideration for a regulatory versus advisory speed limit should be 

whether or not the area under consideration is a stretch of road and not an isolated location. 

Another consideration the highway agency must take into account is the legal authority that its State has 

for posting variable speed limits. It is important in order to ensure that the agency has legal authority to 

set and enforce VSLs. This is elaborated upon in 5.2 Guideline 18: Determine the legal authority to post 

variable speed limits.

Some States choose to use advisory VSL systems for specific reasons. For example, Missouri’s VSL system was 

unresponsive to changing conditions, which resulted in a lack of compliance and acceptance by drivers and law 

YES

NO

Is the area a stretch

of road (typically

at least 2 miles)?

Use Advisory VSL

System

Use Regulatory

VSL System

Figure 3: Regulatory Versus Advisory VSL Flowchart

Figure 4: Example Regulatory and Advisory Variable Speed Limit Signs

13-1057-FHWA_VSL_Report_v3.indd 11 8/30/2012 9:10:17 A
12 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

enforcement, so the system was changed to advisory. Minnesota uses an advisory system because a regulatory 

system would have required changes to State statutes, and the advisory system could be more readily deployed. 

Guideline 3: Consider a semi-automated or automated approach for variable speed limit systems.

Automated and semi-automated speed limit systems use an algorithm that determines what the 

maximum safe speed is based on real-time field conditions. With semi-automated systems, the traffic 

management center (TMC) operator is notified of what the algorithm-recommended speed is, at which 

point the operator has the option of approving or rejecting it. The systems can be designed so that 

the speed limit will be automatically displayed if the operator does not respond within a certain period 

of time. If an automated system is being used, it is important that the credibility of the speed limits be 

checked and modified frequently — annually, at a minimum. 

If it is necessary to operate a VSL in a manual setting, it is important to determine who is responsible for 

activating the speed limits. Although it is not necessary for this responsible party to be TMC personnel, 

that is often the case. Whoever the chosen responsible party is should set and change speed limits 

based on real-time field and weather conditions. It is also important that law enforcement officers and 

other authority figures such as supervisory maintenance personnel have the ability to suggest a speed 

change. This is important because less than ideal conditions may exist on the roadway that the system 

controller, who may be located remotely, is not aware of. 

3.2 Determining Speed Limits for Variable Speed Limit Systems

Guideline 4: Incorporate weather responsive decision support into existing variable speed limit 

algorithms to determine the displayed speed limit.

As discussed in the previous chapters, sight distance and stopping distance are the primary factors to 

consider when setting speed limits. If drivers cannot see the roadway ahead, they may not have time 

to avoid a potential hazard. Drivers traveling at or near the speed limit should have time to make an 

emergency stop before reaching the potential hazard in their driving path. 

A weather responsive algorithm that uses information from traffic, pavement, and weather sensors should 

be used to determine the maximum safe speed limit, based on real-time conditions, which should be 

displayed on the VSL system. The sensor data should provide the algorithm with sufficient information to 

determine a speed limit that will allow for safe sight and stopping distances under the current conditions. 

It is particularly important for a VSL to display speeds that are representative of the conditions, especially 

during less than ideal weather conditions, because drivers trust the VSL to do so. 

At present, there are no generally accepted, comprehensive algorithms for determining appropriate speeds 

for VSLs in wet weather conditions when there are sight distance concerns. For informational purposes, 

an untested sample algorithm is provided in Figure 5 to demonstrate the inputs and the calculations that 

need to be performed, using standard, accepted roadway design equations, as detailed in the rest of this 

section. The Western Transportation Institute is currently performing research for the Oregon Department 

of Transportation (DOT) under the study “Evaluation of a Variable Speed Limit System for Wet and Extreme 

Weather Conditions” to determine a wet weather algorithm. The first phase of this research has been 

completed, which included evaluation of a sensor system to measure the road surface state and depth of 

precipitation on the roadway. This system has proved successful in consistently identifying the roadway 

condition as dry, moist, wet, snowy or icy on both asphalt and concrete. 
| 13

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

The sample algorithm assumes that all speed limits should be set to the next lowest 5 mph increment and 

that the selected speed limit will be no higher than the maximum allowable posted speed and no lower 

than the minimum predetermined speed. It is also assumed that an algorithm may be used on its own 

or may be used as part of an algorithm for VSL systems that are specially designed for other purposes. 

The sample algorithm begins with obtaining speed and pavement condition data and determining if the 

roadway is wet. If the roadway is not wet, the sample algorithm stops and the lesser of the 85th percentile 

operating speed or design speed should be displayed. If the roadway is wet, the sample algorithm 

continues. It is recommended that site-specific coefficients of roadway adhesion be measured for a 

variety of weather conditions, although the sample algorithm can be used whether or not the pavement 

quality is known. Although different studies have shown a variety of values for the coefficient of adhesion, 

if the actual values are not known, one can assume a coefficient of adhesion value, μ, of 0.6 for rain 

conditions and 0.25 for snow or ice for maximum braking on poor pavement, based on past research (13). 

Once sight distance, S, is determined from a field study or from visibility sensors in inclement conditions, 

the sight distance can be used to calculate the displayed VSL speed, V, using μ, S, and G, where V1 

indicates the vehicle speed in feet per second, g refers to the gravitational constant of 32.2 ft/se, ηb refers 

to the braking efficiency as a decimal percentage, μ indicates the coefficient of road adhesion, and G is 

equal to the roadway grade in percent expressed as a decimal (i.e. a downhill grade of -3% is -0.03). This 

relationship is based on an accepted sight distance equation (14) and is expressed as:

where

S = sight distance (ft),

V1 = displayed VSL speed (ft/s),

g = gravitational constant (32.2 ft/se),

ηb = braking efficiency (decimal percentage),

μ = coefficient of road adhesion (unitless),

G = roadway grade (percent expressed as a decimal).

When converting the initial speed V1 from feet per second to miles per hour, setting the gravitational 

constant g equal to 32.2 ft/s2, assuming a value of 1.0 for a vehicle’s braking efficiency factor since antilock 

braking systems are more prevalent on today’s vehicles, and adding in the distance traveled during 

perception-reaction time using the accepted value of 2.5 seconds, the following equations are derived:

Once the VSL display speed is calculated, it can be compared to the current 85th percentile speed to 

determine whether the calculated displayed VSL speed, the current 85th percentile speed, or the design 

speed should be displayed. If the calculated speed is less than the 85th percentile speed, the lesser of 

the calculated speed or design speed should be displayed. If the calculated speed is not less than the 

85th percentile speed, the lesser of the 85th percentile speed or design speed should be displayed. All 
14 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

NO

NO

YES

NO

YES

YES

Calculate Speed, V from:

Obtain speed data and

pavement condition data.

Use determined

coefficient of adhesion

value, μ, for current

weather condition.

Is roadway wet?

Obtain sight distance, S,

from field study or

visibility sensors.

Is pavement

quality known?

Display 85th percentile

operating speed or design

speed, whichever is less*.

Determine roadway

grade, G, in decimal

percentage (-3% = -0.03). 

Assume poor wet pavement,

μ = 0.6 during rain

and μ = 0.25 for snow or ice. 

Is V < 85th

Percentile Speed?

Display V or design

speed, whichever is less*.

Display 85th percentile

operating speed or design

speed, whichever is less*.

Figure 5: Example VSL Algorithm for Wet Weather
| 15

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

speed limits should be set to the next lowest 5 mph increment, but no higher than the maximum allowable 

posted speed limit and no lower than the minimum pre-determined displayed speed (for more details, 

see Guideline 6: For freeways, set a minimum speed limit of not less than 30 mph for regulatory systems or 

not less than 15 mph for advisory speed systems. 

It is important to recognize that there are many variables that can be used for determining an appropriate 

speed based on sight distance. The example equation above uses very conservative values; however, since 

it is very important for a speed limit to be credible, it is necessary to develop and use an algorithm that results 

in reasonable speed limits that motorists will both find credible and respect. Therefore, it is critical to adjust 

the various values for braking efficiency, coefficients of adhesion, and perception-reaction times to those 

appropriate for each specific site. Conditions are rarely the same as those used in the sample algorithm above. 

In addition to algorithms that calculate an appropriate speed limit to be displayed, some VSL systems use 

a pre-determined speed reduction for wet weather or poor surface conditions. This speed reduction can 

either be one pre-set speed that the system displays any time conditions are less than ideal, or it can be 

a range of speed limits that the system can display. The latter is recommended in order to incorporate 

adjustments for varying weather conditions. 

Guideline 5: All speed limit algorithms and manual display calculations should be approved by a traffic 

engineering professional. 

Algorithms and manual display determinations should be established based on a traffic engineering 

study, even if speed limits are manually adjusted by a TMC operator. It is not acceptable for a TMC 

operator to change and set VSLs arbitrarily. 

Guideline 6: For freeways, set a minimum speed limit of not less than 30 mph for regulatory systems or 

not less than 15 mph for advisory speed systems. 

It is important to pre-set a minimum speed limit on a VSL system. Many international agencies use a 

minimum speed limit of 30 mph for VSL systems, whereas some State agencies within the United States 

have set a minimum speed limit of 40 mph, or no more than 20 mph under the normal posted speed limit. 

Advisory VSL speed limits have been used in the United States with minimum displayed speeds of 15 mph. 

Guideline 7: Use speed limits in 5 mph increments.

It is advisable that VSL systems be designed with 5 mph speed increments. The majority of the existing VSL 

systems are currently successfully using 5 mph hour speed increments. Speed increments smaller than 5 

mph will likely confuse drivers, as speed limits are always posted in multiples of 5 mph. Increments greater 

than 5 mph will not provide drivers with an accurate, safe speed limit. 

3.3 Determining Display and Location of Variable Speed Limit Signs

Guideline 8: Display variable speed limit changes for at least 1 minute. 

Speed limits should be updated frequently enough that they represent real-time conditions, but not so 

frequently that they confuse or overwhelm drivers. For automated systems, speed recommendations 

should be generated and sent to the VSL operator at intervals of 60 seconds or less so that the system is 

generating speed recommendations based on current conditions. However, for all VSL systems, speeds 

should be displayed for at least 1 minute so that approaching drivers are not confused by seeing more 

than one speed limit change. 
16 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

Guideline 9: Do not display reduced speed limits more than 1 mile upstream from the section of 

roadway where the reduced speed is desired.

If the distance between the display and the 

speed reduction is too short, drivers will not 

have enough time to react. On the contrary, 

if the distance between them is too long, 

motorists will likely resume normal speeds 

before they reach the area where speed 

reduction is necessary. Credibility and 

responsiveness are the keys to a successful 

VSL system. VSL signs should be spaced 

at regular intervals where conditions are 

likely to vary. In Figure 6, a VSL system has 

been in place on the New Jersey Turnpike 

for decades that is capable of showing 

reduced speed limits due to ice, snow, fog, 

construction, accidents, or congestion. A 

replacement plan is currently in place to 

replace the signs, but changeable message 

signs will still be used with VSLs to adjust for 

various roadway conditions. If the congestion 

warning shown in the figure with the reduced speed limit of 45 mph is shown too far upstream of the 

congestion, motorists will begin to ignore the signs and will likely accelerate. 

Guideline 10: Where variable speed limit signs are closely spaced, do not allow speed differentials of 

more than 15 mph without advance warning.

When VSLs are used in a corridor, it is important for there to be multiple displays throughout the corridor. 

These signs should not all automatically display the same speed; each speed displayed should be based 

on the real-time conditions at that location. This may require additional sensors, but it is an important 

design factor because rain intensity can vary greatly throughout a long corridor. However, it is important 

for VSLs that are less than 1 mile apart to not display speeds that are drastically different unless advance 

warning is provided of the reduced speed ahead. In the United States, one agency will not allow two 

consecutive VSLs to differ by more than 15 mph.

Guideline 11: Use Changeable Message Signs (CMS) to communicate reasons for speed reduction.

When possible, any additional data that can increase the credibility of the system should be included 

on a CMS, either attached to the VSL itself or nearby. If there are CMSs located near the VSL system, 

they should be used to communicate reasons for the reduced speeds. Some examples of messages 

to be displayed include “CAUTION RAIN AHEAD,” “SLOW TRAFFIC AHEAD,” “STOPPED TRAFFIC AHEAD,” 

“XX MPH TRAFFIC AHEAD,” or “REDUCED SPEED LIMIT AHEAD.” For further information about acceptable 

messages to display on CMSs, refer to Chapter 2L of the MUTCD (11). Legislation in some States may 

require a message sign indicating the reason for a speed change. Another good use for a CMS in 

coordination with a VSL system is to warn drivers entering the roadway that VSLs are being used. Figure 

7 shows an example from Wyoming on Interstate 80 of how a sign can be used to warn of changing 

road conditions. 

Figure 6: Variable Speed Limit System on the New Jersey Turnpike 

(Source: Bryan Katz)
| 17

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

Guideline 12: As with static speed limit signs, variable speed 

limit signs should be placed at all entrances to the roadway.

Additional speed limits should be installed beyond 

intersections and interchanges to remind users of the speed 

limit as well as to provide information to traffic entering from 

other roadways.

Additional information and guidance on communicating road 

weather messages, including VSL displays, can be found in 

http://ntl.bts.gov/lib/33000/33000/33047/rev_final_hf_analysis_

road_weather.pdf. 

Chapter 4. Guidelines for the Design, 

Installation, Operation, and Maintenance of 

Wet Weather Variable Speed Limit Systems

This chapter provides guidelines for developing a comprehensive 

concept of operations for a VSL system, determining where 

weather sensors are appropriate, ensuring that all necessary 

equipment is incorporated in the requirements, and developing 

an operations and maintenance plan. 

4.1 Guideline 13: Develop a comprehensive concept of operations for the 

variable speed limit system.

Once the decision has been made that a VSL is appropriate for a location, the first step in the design 

process should be to develop a concept of operations. A concept of operations is a document that helps 

ensure that the system contains all required features. This document will describe the characteristics 

of the proposed system and may involve the full system engineering process for developing it. FHWA’s 

Systems Engineering Guidebook for ITS can be used as a guide in this process (15). In the operational 

concept, it is important to keep issues related to system operations and maintenance in mind and not to 

lose sight of the primary purpose of a VSL system, which is to reduce speed variance and avoid accidents 

caused by drivers traveling at an unsafe speed.

4.2 Guideline 14: Install appropriate weather sensors or use accurate weather 

data at problem locations.

It is important to determine the data needs of a VSL system, which data sources should be used to meet 

these needs, and when they should be used to ensure that adequate data is collected. The data that 

should be collected for wet weather VSLs are traffic, weather, and pavement conditions. Either real-time 

traffic cameras or sensors can be used to collect traffic data, but data on weather conditions can be 

collected in a variety of ways, including video, pavement sensors, weather radar, and sensors for rain and 

fog. Additionally, it is important for personnel on the ground, such as law enforcement, first responders, 

and other incident management personnel, to be able to request speed reductions based on their 

observations in cases where there are weather conditions that the system operator is not yet aware. 

Figure 7: Changeable Message Sign in Wyoming Used 

to Communicate Speed Reduction Warning 

(Source: Bryan Katz)
18 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

4.3 Guideline 15: Ensure that equipment required in the design process is 

incorporated into the installation requirements.

In order to avoid potential issues, there are a variety of details that need to be considered when designing 

any VSL system. These details include sensors, communications, power, operator confirmation and 

override, and a police notification process. When designing a VSL system, it is important to have adequate 

speed and visibility sensor coverage in the vicinity of the VSL sign. The most cost-effective time to add 

these sensors is when power and communication are being added at the beginning of the design phase. 

Although sensors may be costly to begin with, they are even more expensive to add after the system is 

designed. It is important to budget for and include a reasonable number of sensors in advance. 

The communications and power for a VSL system should be designed in conjunction with other nearby 

devices that use similar technology, such as Closed Circuit Television (CCTV). If running landline 

communications and power is cost prohibitive, wireless communication and solar power may be more 

cost-effective options. However, it is important to consider overall power requirements and whether solar 

energy will provide enough power to the entire system. Additionally, cost and maintenance of batteries 

should be considered since battery life may be reduced in harsh environments. Additional technology 

that should be included in the design of a VSL system are speed detectors, rain sensors, Bluetooth 

detectors, and a method to provide police notifications. It is crucial for police officers to be aware of 

the speed limit that a VSL is displaying at any given time. Because police officers typically take their 

communications gear with them when shifts change, it is important that the notifications are broadcast 

and not sent directly to individual officers. 

Another important detail to ensure is that the system outputs information into a log of all actions that the system 

makes. Regardless of whether changes in the system are made automatically or by the operator, it is extremely 

important to maintain a log. Any changes made to the speed display should be logged, as should the source 

of the change. This log should be designed in a way that ensures the data will be archived and backed up. 

4.4 Guideline 16: Develop an Operations and Maintenance Plan.

A plan for operating and maintaining the VSL system should also be included in the process. It is easy for 

maintenance to be overlooked when budgets are being created, but it is a necessary element. To maintain 

both driver safety and driver trust in the system’s accuracy, it is important that a VSL system operate properly 

and reliably. Agencies should develop plans in advance for regular cleaning, checking, and calibration, 

not just of the signs, but of all the equipment, including road weather and traffic sensors, cameras, and 

communications. This should be at least a 5-year plan and include information on funding requirements. 

Additional studies should be completed after the VSL has been implemented in order to measure the 

effectiveness of the system. If these studies show that the VSL system is not operating effectively, changes may 

need to be made regarding either the system itself or related enforcement efforts. Guidance for developing 

operations and maintenance manuals can be found in FHWA’s Systems Engineering Guidebook for ITS (16). 

Important elements that are relevant to VSLs and should be clearly addressed in the plan include:

•	 Identification of the personnel responsible for operation and maintenance; 

•	 Identification of the human resources and facilities, including tools, needed for operation and 

maintenance; 

•	 Identification of funding sources for on-going operation and maintenance;
| 19

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

•	 Descriptions of the operation and maintenance activities to be performed;

•	 Descriptions of the checks to be made, and the data to be collected, for system health and 

performance monitoring;

•	 Necessary training for operators and maintenance personnel;

•	 Identification of other documents used in operations and maintenance, such as relevant policy 

directives, system configuration documentation, and operation and maintenance manuals; 

•	 Preventive as well as reactive maintenance activities; and

•	 Life expectancy of the system and end-of-life replacement or upgrade processes. 

Chapter 5.Guidelines for the Enforcement of Wet Weather Variable Speed 

Limit Systems

The following guidelines pertain to coordinating regulatory VSL system installation and operation with law 

enforcement and the judicial system. This includes guidance on coordinating and collaborating with law 

enforcement, determining legal authority, and maintaining a system log for evidence of potential violations.

5.1 Guideline 17: Coordinate and collaborate with law enforcement before 

deploying a variable speed limit system.

The State highway and law enforcement agencies should work together closely to determine how the VSL 

system will operate and be enforced. An interagency agreement should be developed that specifies the 

standard operating procedures to be followed and the respective responsibilities of the agencies. The 

agreement should address items such as:

•	 Delegation of authority for setting and changing the VSL;

•	 How the VSL will be set, including minimum and maximum speeds to be posted;

•	 How often the speed limit can change;

•	 How the VSL will be enforced;

•	 How best to communicate speed limit changes to patrol officers; and

•	 How law enforcement can access VSL sign displays and logs.

For any VSL to be successful in effectively managing speed and reducing crash risk, there must be a clear 

vision of how the system will operate and be enforced, and all participants must share this vision. Engineers, 

police, and the courts must know their roles and responsibilities as they relate to VSL. The potential benefits 

of this coordination and collaboration are as follows: a coordinated approach that supports the integrity of 

enforcement; an enforcement threat that deters inappropriate speeds and encourages compliance with VSL; 

speed violations successfully adjudicated; and increased safety for officers writing citations.

A map-based web page, text messages, email, or other methods could be used to notify patrol officers of 

current speed limits as well as the date and time of last change. A grace period should be allowed after 

each speed limit change before commencing enforcement of lower speed limits to ensure that the speed 

limit at time of the violation was seen by passing motorists.
20 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

5.2 Guideline 18: Determine the legal authority to post variable speed limits.

The highway agency will need to identify all jurisdictions responsible for speed enforcement in the zone 

with VSLs. This is important in order to ensure that the agency has legal authority to set and enforce VSLs 

so that speed violations can be successfully adjudicated. The State highway agency and State police 

must work with State legislators and council members in local jurisdictions to ensure that the necessary 

statutes are enacted or revised, as needed, to make VSLs enforceable. 

Most, if not all, States have a speed zoning statute that delegates to the DOT the power to establish or 

change speed limits on the basis of an engineering investigation. The States that have implemented VSLs 

have done so mainly under the broad authority provided in this speed zoning provision of State Law or 

through a special provision in its vehicle code to allow for VSL systems. 

Speed limits are established by statute or on the basis of an engineering study. Section 11-803 of the 

Uniform Vehicle Code (17) also includes the following provision:

Such a maximum speed limit may be declared to be effective at all times or at such times 

as are indicated upon the said signs; and differing limits may be established for different 

times of day, different types of vehicles, varying weather conditions, and other factors 

bearing on safe speeds, which shall be effective when posted upon appropriate fixed or 

variable signs.

This provision specifically supports the setting of VSLs and should be incorporated into the State 

speed zoning statute. Also, the Judicial Enforcement of Variable Speed Limits report addresses legal 

considerations for implementing and enforcing VSLs and should be referred to for more detailed 

information on this topic (18). 

5.3 Guideline 19: Maintain system log for evidence of speed limit violations.

A system log should be maintained that provides evidence of the speed limit posted at time of the offense, 

as well as the prevailing traffic speeds, environmental conditions, or other reasons for any speed limit 

change. A copy of the engineering study documenting the decision logic and need for VSL at a particular 

location should be readily available to law enforcement and the courts.

For speed violation to be successfully adjudicated, the State should provide certifiable evidence of the 

speed limit posted at time of the violation and the reason the speed limit was changed. Speed limits that 

differ from the general statutory speed limit must be based on an engineering investigation. The potential 

benefits of maintaining a system log are as follows: successful adjudication of speed violations; fair and 

reasonable speed management system, as perceived by drivers; greater respect for law enforcement; and 

enforcement targeted at drivers that disproportionately contribute to crash risk. The system logs could also 

be used as part of a continuous ongoing evaluation to monitor and determine the effectiveness of the VSL 

system and changes needed to improve its effectiveness.

Chapter 6.Weather-Related Variable Speed Limit Case Studies

In order to provide an in-depth analysis of the VSL systems and to obtain additional information on their 

use in wet weather and issues related to sight distance and stopping distance, various States with active 

VSL systems were interviewed. These States were chosen from the list of known U.S. installations of VSLs, 
| 21

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

which can be found in Appendix B, based on their relevance to the purpose of this guide, the level of 

experience each governing agency had with VSL systems, and the agency’s willingness to share detailed 

information about their systems. The States below have VSL systems that currently incorporate weather 

conditions in their speed-setting criteria.

6.1 Alabama

Alabama DOT (ALDOT) currently operates a VSL system on a 7-mile section of Interstate 10 in Mobile, 

Alabama. The section of roadway where the VSL system is implemented previously had a very high 

number of vehicle accidents due to visibility issues caused by fog. Following a very severe crash in 1995 

involving 193 vehicles, ALDOT chose to deploy this low visibility warning system. 

The VSL system collects data from remote vehicle detectors, fog detection devices, and visibility sensors. The 

visibility sensors use forward-scatter technology and are installed roughly every mile. The data is reviewed 

by the TMC operators, who then manually change the speed limit based on the existing weather and traffic 

conditions. The operators use charts that detail what the posted speed limit should be based on driver 

visibility. The speed limits are changed in increments of 10 mph within the range of 35 to 65 mph. The 

system controls a total of 24 VSL signs, but is divided into six zones, in which the speeds in each zone can be 

set independently. Table 1 below shows the speeds and other strategies based on visibility distance (19). 

Table 1: Alabama DOT Low Visibility Warning System Strategies

Visibility Distance Speed Limits and Other Strategies

Less than 900 feet (274.3 meters) Speed limit at 65 mph (104.5 kph)

Less than 660 feet (201.2 meters) Speed limit at 55 mph (88.4 kph)

Less than 450 feet (137.2 meters) Speed limit at 45 mph (72.4 kph)

Less than 280 feet (85.3 meters) Speed limit at 35 mph (56.3 kph) and street 

lighting extinguished

Less than 175 feet (53.3 meters) Road Closure by Highway Patrol

Source: Best Practices for Road Weather Management, Version 2.0, U.S. Department of Transportation, 

Federal Highway Administration, Washington, D.C., May 2003, http://ops.fhwa.dot.gov/weather/best_

practices/CaseStudiesFINALv2-RPT.pdf (accessed May 31, 2012).

The system is set up to notify local and State law enforcement immediately of any changes to the conditions 

and speed limits on I-10. The system allows the TMC to notify the public of the existing conditions and the 

appropriate speed limit. The public perception of the system is positive because ALDOT has taken steps 

toward dealing with the issues with fog. There are plans to extend the corridor farther on I-10 and onto I-65. 

6.2 Delaware

Delaware DOT (DelDOT) currently operates a VSL system on Interstate 495 that changes speed limits 

based on weather conditions. The speed limits are varied by DelDOT operators at the TMC either by 
22 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

decision of the Chief Traffic Engineer, delegated to the TMC Manager, or at the request of the Delaware 

State Police (DSP) for extreme weather conditions and poor roadway surface conditions. 

Real-time traffic conditions such as speed, volume, and occupancy are collected at count stations along the 

roadway, and weather data is obtained from cameras and personnel in the field. Speed reductions of 5 to 

20 mph can be made for any of the following conditions: traffic incidents, extreme weather conditions (heavy 

precipitation, high winds, reduced visibility due to precipitation, fog, or smoke), and poor roadway surface 

conditions (ice and/or snow covered pavement, “black ice” patches on pavement material, and object spills 

on the roadway pavement or roadsides). Speed limits are not adjusted for ozone action days, as was initially 

envisioned. The concern was that motorists cannot perceive a reason why they need to slow down, so using 

the VSL system on ozone action days may actually create speed differential issues along the roadway. 

The VSL system is regulatory, but the DSP maintains the authority to issue tickets to drivers when the issuing 

officer believes they are traveling too fast for conditions, regardless of the posted VSL. As per Delaware 

code, drivers have a responsibility to adjust their speeds based on the prevailing conditions regardless 

of the static signing. The same speed limit is typically displayed throughout the entire corridor, but it is 

possible for the signs to display different speeds. 

The VSL system has been very favorably received by police, emergency services, and DelDOT personnel 

involved with maintenance or construction projects along the roadway. Other than some initial reliability 

issues with communications, DelDOT has received no negative feedback on the signs. Over time, their 

goal is to install similar systems along all of the interstates and limited access facilities. 

6.3 South Carolina

South Carolina DOT (SCDOT) launched its VSL project in 2009 as the result of a Road Safety Audit (RSA) 

that determined that a VSL system was necessary to decrease the number of speed and wet weather 

related crashes on US 25 in Greenville County. According to SCDOT, 85 percent of accidents between 

2003 and 2007 occurred during wet weather conditions, and speed was a contributing factor in 62 of 

these 71 accidents (20). The system covers a 2-mile rural segment of the roadway that has a 55 mph 

speed limit and a 6 percent downgrade. The project was delayed due to power issues, but the system 

has been operational since mid-2011. 

The system automatically reduces the speed limit to 45 mph if non-ideal roadway conditions are present. 

Currently this is determined based on the amount of precipitation, which is collected by a standard 

weather station installed nearby. Sensors are installed in the pavement to read barometric pressure and 

determine if ice is forming on the pavement. Cameras are linked to the central system so that SCDOT 

can watch the VSLs. The VSL signs are supplemented by overhead variable message signs that display 

standard messages based on the type of weather condition being experienced. 

6.4 Washington

Washington State DOT (WSDOT) currently has several VSL systems in place on the following roadways: I-5 

Northbound into downtown Seattle, I-90 between the City of Bellevue and Seattle, SR 520 between the City 

of Bellevue and Seattle, US 2 over the Stevens Mountain Pass, and I-90 over the Snoqualmie Mountain Pass. 

All of the speed limits posted on VSL systems in Washington are regulatory and enforceable. The VSLs in the 

urban areas are set automatically based on speed and occupancy sensors correlated to an 85th percentile 

speed, so wet conditions only affect speed selection if traffic is driving slowly because of the conditions. 
| 23

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

However, the speeds in the mountain passes are set manually based on environmental sensor information. 

WSDOT implemented this speed management system on I-90 through Snoqualmie Pass, a popular tourist 

destination, in response to a high crash rate brought about by the rain and fog of summer and the snow 

and ice of winter. The system collects data from 6 environmental sensor stations (ESS) and 22 radar 

vehicle detectors. The ESS detect a variety of environmental information, including air temperature and 

humidity, precipitation, wind speed, and pavement temperature and condition. 

All of the data is sent to a central control computer that calculates the suggested safe speed limit based 

on the strategies provided in Table 2. If the system operators agree with the suggested speed, the VSL 

signs are activated to display the reduced speed limits and the CMSs are activated to display the reason 

for the reduced speeds and the road weather advisories. The roadway is broken down into sections by 

direction, so that the operators can modify the speeds in each section independently from the others. 

The speed limits can vary between 35 mph and 65 mph in 10 mph increments (19). 

A study conducted by the University of Washington found that the speed management system reduced the 

average vehicle speed by up to 13 percent. Speed variance increased slightly, but the overall roadway safety 

has improved as drivers are required to decrease speed during non-ideal weather and roadway conditions (19). 

Table 2: Washington State DOT Speed Management Control Strategies

Weather Conditions Pavement Conditions Control Strategies

•	 Light to moderate rain

•	 Visibility distance greater 

than 0.5 mi. (0.80 km)

•	 Dry

•	 Wet

•	 Speed limit remains at 65 

mph (104.5 kph)

•	 No tire regulations

•	 Heavy rain

•	 Fog

•	 Visibility distance less than 

0.2 mi. (0.32 km)

•	 Slushy

•	 Icy

•	 Speed limit reduced to 55 

mph (88.4 kph)

•	 Traction tires advised

•	 Heavy rain or snow

•	 Blowing snow

•	 Visibility distance less than 

0.1 mi. (0.16 km)

•	 Shallow standing water

•	 Compacted snow/ice

•	 Deep slush

•	 Speed limit reduced to 45 

mph (72.4 kph)

•	 Traction tires required

•	 Freezing rain

•	 Heavy rain or snow

•	 Blowing snow

•	 Visibility distance less than 

0.1 mi. (0.16 km)

•	 Deep standing water

•	 Deep snow/slush

•	 Speed limit reduced to 35 

mph (56.3 kph)

•	 Tire chains required

Source: Best Practices for Road Weather Management, Version 2.0, U.S. Department of Transportation, 

Federal Highway Administration, Washington, D.C., May 2003, http://ops.fhwa.dot.gov/weather/best_

practices/CaseStudiesFINALv2-RPT.pdf (accessed May 31, 2012).
24 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

6.5 Wyoming

In February 2009, Wyoming DOT (WYDOT) implemented a VSL system along the Elk Mountain Corridor, 

which is located in southwestern Wyoming on Interstate 80 between Laramie and Rawlins. This corridor 

is used by both private citizens and commercial carriers, and several stretches experience severe winter 

weather. The corridor was previously equipped with weather stations and CMS. The original system was 

equipped with 20 VSLs and was expanded by 8 additional VSLs during the 2009–2010 winter seasons. 

Weather conditions and visibility are monitored by cameras and road and wind sensors, surface and 

atmospheric conditions are collected at the weather station located in the middle of the corridor, and 

speed is monitored by sensors in the pavement and markers alongside the highway. 

The VSL system currently consists of 140 miles in four segments that cover approximately 35 percent 

of Interstate 80. The Wyoming Highway Patrol, maintenance foremen, and TMC operators are 

responsible for speed selection. Visibility, surface conditions, and vehicle speeds are used to determine 

appropriate speed limits. Wyoming Highway Patrol visually inspects conditions and reduces speed limits 

as deemed appropriate. The maintenance foreman can also lower speeds if the Wyoming Highway 

Patrol is not available. If the average vehicle speed drops 15 mph from the standard speed and no one 

else is on the corridor to confirm conditions, then the TMC Operator has the authority to reduce speed 

limits. Also, if there is a drop in visibility or a change in surface conditions, the TMC Operator has the 

authority to set the speed limits based on the criteria provided in Table 3 (21).

Table 3: Wyoming DOT Speed Management Strategies

Reported Condition Wet or Dry Slick Spots Slick

Speed Limit (mph) Visibility (feet)

75 >950 >1625

65 735-950 1225-1625 >1700

50 475-725 750-1225 1025-1700

35 <475 <750 <1025

Source: “Wyoming Department of Transportation Policy Memorandum: Variable Speed Limit,” Wyoming 

Department of Transportation, GIS/ITS, Cheyenne, WY, February 16, 2011. 

In urban areas, the system primarily monitors incidents and vehicle speeds; however, in rural areas, the 

system monitors visibility, weather, and pavement conditions. There is no limit to how often speed limits 

may be changed or how long the speeds must be displayed. 

Legislation for WYDOT’s VSL system went into effect in July 1, 2008. This legislation grants authority to set speed 

limits based on “vehicle or weather emergency.” The legislation also states that speed limits can vary by time 

of day, vehicle type, weather conditions, and other factors, which can be posted on fixed and variable signs.

6.6 Discussion

Although several VSL systems are currently being used or have been used in the past in the United States, 
| 25

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

enhancements can be made in decreasing speed limits for wet weather conditions. Many States base 

their VSLs on current vehicle speeds, so wet weather does not affect speed changes unless traffic is 

driving slowly because of the conditions. For example, in Washington, environmental sensor information 

is considered when making speed decisions in the mountain passes, but not in the urban areas. In 

Minnesota, the possibility of incorporating rain sensor data and other weather condition information into 

their algorithm is being researched, but the algorithm is currently based on difference in speed along the 

corridor. Missouri has not explicitly considered rain and snow in their speed selection process, except in 

very extreme circumstances where a specific hazard is known. Typically, the system responds to weather 

conditions based on real-time traffic conditions. 

These examples show the common theme that exists among most VSL systems, reflecting a need for wet 

weather enhancements. Because of this, the NTSB issued safety recommendation H-05-14, requesting 

that guidance be issued recommending the use of VSL signs in wet weather at locations where the 

operating speed exceeds the design speed and the stopping distance exceeds the available sight 

distance. In response to this NTSB recommendation, this document provides guidelines developed from 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. 

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. 
26 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

Appendix A. References

1. McGovern, C., P. Rusch, and D. Noyce, State Practices to Reduce Wet Weather Skidding Crashes, 

FHWA-SA-11-21, U.S. Department of Transportation, Federal Highway Administration, Washington, 

D.C., May 2011, http://safety.fhwa.dot.gov/speedmgt/ref_mats/fhwasa1121/fhwasa1121.pdf.

2. A Policy on Geometric Design of Highways and Streets, 6th Edition, American Association of State 

Highway and Transportation Officials, Washington, D.C., 2011.

3. Warrants for the Installation and Use of Technology Devices for Transportation Operations and 

Maintenance, FHWA ENTERPRISE Pooled Fund Study, http://acconsultants.org/itswarrants/forms/vsl.

html (accessed on April 17, 2012).

4. Lunenfeld, H. and G.J. Alexander, A User’s Guide to Positive Guidance, 3rd Edition. U.S. 

Department of Transportation, Federal Highway Administration, Washington, D.C., September 1990.

5. Traffic Engineering Handbook, 6th Edition, Institute of Traffic Engineers, Washington, D.C., 2009. 

6. Campbell, J.L., C. M. Richard, and J. Graham, Human Factors Guidelines for Road Systems. NCHRP 

Report 600A, Transportation Research Board of the National Academies, Washington D.C., 2008.

7. Highway Statistics 2010, Licensed Drivers, Vehicle Registrations, and Resident Population (In 

Millions), U.S. Department of Transportation, Federal Highway Administration, Washington, D.C., 

http://www.fhwa.dot.gov/policyinformation/statistics/2010/ (accessed on April 17, 2012). 

8. Potts, I., J. Stutts, R. Pfefer, T. R. Neuman, K.L. Slack, and K.K. Hardy, NCHRP Series 500 Guidance 

for Implementation of the AASHTO Strategic Highway Safety Plan. Volume 9: A Guide for Reducing 

Collisions Involving Older Drivers, Transportation Research Board of the National Academies, 

Washington D.C., 2004.

9. Wet Weather Accident Reduction Program (WWARP), Texas Department of Transportation, Austin, 

Texas, 2006.

10. Hall, J.W., K.L. Smith, L. Titus-Glover, J.C. Wambold, T.J. Yager, and Z. Rado, Guide for Pavement 

Friction, NCHRP Web-Only Document 108, Transportation Research Board of the National 

Academies, Washington D.C., 2009.

11. 2009 Manual on Uniform Traffic Control Devices, Federal Highway Administration, U.S. Department 

of Transportation, December 2009, http://mutcd.fhwa.dot.gov/. 

12. Survey of Speed Zoning Practices, Institute of Transportation Engineers, Washington, D.C., 2001.

13. Shadle, S.G., L.H. Emery, and H.K. Brewer, Vehicle Braking, Stability, and Control, SAE Transactions, 

Vol. 92, paper 83062, 1983.

14. Mannering, F.L. and S.S. Washburn, Principles of Highway Engineering and Traffic Analysis, Fifth 

Edition, John Wiley & Sons, Hoboken, NJ, 2013. 

15. Systems Engineering for Intelligent Transportation Systems: An Introduction for Transportation 

Professionals. Federal Highway Administration, U.S. Department of Transportation, January 2007, 

http://ops.fhwa.dot.gov/publications/seitsguide/seguide.pdf.
| 27

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

16. Systems Engineering Guidebook for ITS, V3.0, Federal Highway Administration, U.S. Department of 

Transportation, January 2007, http://www.fhwa.dot.gov/cadiv/segb/views/document/sections/

Section8/8_4_12.htm. 

17. Uniform Vehicle Code. National Committee on Uniform Traffic Laws and Ordinances. 

18. Judicial Enforcement of Variable Speed Limits, Legal Research Digest No. 47, National Cooperative 

Research Program, March 2002, http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_lrd_47.pdf.

19. Best Practices for Road Weather Management, Version 2.0, U.S. Department of Transportation, 

Federal Highway Administration, Washington, D.C., May 2003, http://ops.fhwa.dot.gov/weather/

best_practices/CaseStudiesFINALv2-RPT.pdf (accessed May 31, 2012).

20. South Carolina: Decrease Hydroplaning on US 25 in Greenville County, University of Minnesota Rural 

Highway Safety Clearinghouse, http://www.ruralsafety.umn.edu/clearinghouse/rsip/southcarolina/

(accessed March 28, 2012).

21. “Wyoming Department of Transportation Policy Memorandum: Variable Speed Limit,” Wyoming 

Department of Transportation, GIS/ITS, Cheyenne, WY, February 16, 2011. 
28 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

Appendix B. VSL Installations in the United States

Known installations of VSLs in the United States are presented below. For each installation the following 

information is provided: location, type of activation (manual, automated, hybrid), total length, type 

(regulatory, advisory), number of VSL signs, type of sensors, and current status. The installations are 

sorted first by current status, beginning with the active installations, and then by alphabetical order of the 

State in which the installation resides. 

Known U.S. Installations of VSLs

Location Type of 

Activation

Total 

Length

Type Number of 

VSL Signs

Type of Sensors Current 

Status

I-10, Mobile, 

Alabama

Manual 7 mi Regulatory 24 Visibility, CCTV Active

I-70, Colorado Manual 19 mi Regulatory 8 Loops, Radar, 

Temperature, 

Precipitation, 

Wind speed

Active

I-70, Colorado Manual 1 mi Regulatory 4 Active

Delaware Manual 4 bridges Regulatory 5–8 Speed, Volume, 

Occupancy, 

Environmental 

(weather)

Active

I-4, Florida Hybrid 10.5 mi Regulatory 20 Loop detectors, 

Side-fire radar 

(volume, speed, 

occupancy), 

Weather visible by 

CCTV

Active

I-95, Maine Manual ~195 mi Advisory 65 total 

(solar 

powered)

Some have 

cameras, Radar 

(to read speed of 

traffic)

Active

I-295, Maine Manual 50 mi Advisory Some have 

cameras, Radar 

(to read speed of 

traffic)

Active

I-35W, 

Minneapolis

Automated 15 Advisory 174 Single loops Active
| 29

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

Location Type of 

Activation

Total 

Length

Type Number of 

VSL Signs

Type of Sensors Current 

Status

I-270, St. Louis, 

Missouri

Hybrid 38 mi Advisory 70 (all 

solar

powered)

Speed, 

Occupancy

Active

Turnpike, New 

Jersey

Manual ~148 mi Regulatory 164 Speed 

(Environmental 

sensors are 

inactive due to lost 

connection)

Active

Turnpike, 

Pennsylvania

Manual 10 mi Regulatory 18 Speed, 

Environmental, 

CCTV

Active

Bridges/

Tunnels, 

Virginia

Manual 4 bridgetunnelsRegulatory 50 CCTV Active

I-75, 

Tennessee

Manual 19 mi Regulatory 10 Speed, 

Environmental 

(fog)

Active

I-90, 

Washington

Manual 28 mi Regulatory 14 Speed, 

Environmental

Active

US 2, 

Washington

Manual 23 mi Regulatory 8 Speed, 

Environmental

Active

Seattle 

Metropolitan 

Area, 

Washington 

(I-5, I-90, SR 

520)

Automated Unknown Regulatory Unknown Speed, 

Environmental

Active

I-80 Wyoming Manual 140 mi Regulatory Approx. 42 Speed, 

Environmental

Active

I-84, Idaho Manual 105 mi Advisory 5 Vehicle, 

Environmental

Test Site

I-94, 

Minneapolis

Automated 8 mi Advisory 110 Single loops Under 

construction
30 | Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

Location Type of 

Activation

Total 

Length

Type Number of 

VSL Signs

Type of Sensors Current 

Status

I-77, south of 

I-81, Virginia

Hybrid 20 mi Regulatory 40 TBD Planned

Turnpike/I-595, 

Florida

Automated 2-lane offrampAdvisory 1 Moisture Removed

I-10/I-310, 

Louisiana

Manual Unknown Advisory Approx. 12 Visibility, Speed Removed

I-695, 

Maryland

Automated 3 mi Regulatory 4 Speed, Queue Removed 

(temporary 

installation)

I-96, Michigan Automated 8 mi Regulatory 7 Speed Removed 

(temporary 

installation)

I-494, 

Minnesota

Automated 2.5 mi Advisory 3 Speed Removed

I-80, Nevada Manual 2–3 mi Regulatory 4 Visibility Removed 

(technical 

issues)

I-40, New 

Mexico

Automated 3 mi Regulatory 3 Speed, 

Environmental

Removed

I-526, South 

Carolina

Manual 2 mi No speed 

change

8 Fog Removed

I-80, Utah Manual 6 mi Regulatory 2 Day/Night 

automatic

Removed 

(temporary 

installation)

I-215, Utah Manual 2 mi Regulatory 2 Speed, 

Environmental

Removed

I-95, Virginia 

(work zone 

at Woodrow 

Wilson Bridge)

Hybrid 1 bridge Regulatory Unknown Speed, Queue 

length

Removed
| 31

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather
For More Information:

http://safety.fhwa.dot.gov

FHWA, Offi ce of Safety

Guan Xu

guan.xu@dot.gov

202-366-5892

FHWA-SA-12-022