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FHWA Home / Safety / Speed Management / Guidelines for the Use of Variable Speed Limit Systems in Wet Weather

Chapter 6. Weather-Related Variable Speed Limit Case Studies

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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, 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 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. 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. Available at http://ops.fhwa.dot.gov/weather/best_practices/CaseStudiesFINALv2-RPT.pdf. Accessed May 31, 2012.

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, 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.

Page last modified on October 15, 2014.
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