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FHWA Highway Safety Programs

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).

VSL flowchart.
Figure 2: Wet Weather Variable Speed Limit Flowchart

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 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 real-time data and to be credible to most drivers. This is particularly important when using VSLs 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.

VSL flowchart.
Figure 3: Regulatory Versus Advisory VSL Flowchart

A regulatory VSL sign and advisory VSL sign for a speed limit of 45 mph.
Figure 4: Example Regulatory and Advisory Variable Speed Limit Signs

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 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.

VSL flowchart and algorithm.
Figure 5: Example VSL Algorithm for 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:

Equation: S = V subscript 1 squared divided by 2g(eta beta mu plus or minus G).

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 (14), the following equations are derived:

Equation: S equals the quotient of 0.03V squared divided by mu plus or minus G plus 3.67V.


Equation: (-3.67 plus the square root of 13.47 plus the product of 0.12 divided by mu plus or minus G multiplied by (S)) divided by the quotient of 0.06 divided by mu plus or minus G.

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 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.

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.

Photo of a variable Reduce Speed sign and a VSL sign mounted over one side of a divided highway.
Figure 6: Variable Speed Limit System on the New Jersey Turnpike (Source: Bryan Katz)

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.

Photo of a warning sign with mounted warning lights displaying Caution, a changeable message panel, and Ahead.
Figure 7: Changeable Message Sign in Wyoming Used to Communicate Speed Reduction Warning (Source: Bryan Katz)

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