Weather Applications > Traffic Control in Adverse Weather

• Introduction
  • What are Traffic Control Systems
    for Adverse Weather Conditions?
  • The Rationale for Traffic Control Systems
    for Adverse Weather Conditions
• System Description
  • VMS and Weather
  • Intelligent Speed Adaptation (ISA) and Weather
  • Additional Components of Traffic Control Systems
  • How are Traffic Control Systems for
    Adverse Weather Integrated with other ITS Technologies?
• Assessment
  • Key Results
  • Benefits
  • Costs
  • Implementation and Operational Challenges
• Where are Traffic Control Systems for Adverse
  Weather Conditions Implemented?
• Case Studies
• References

INTRODUCTION

What are Traffic Control Systems for Adverse Weather Conditions?

Traffic control systems for adverse weather are designed to increase road safety by directing traffic flow and speeds. Variable Message Signs (VMS), Highway Advisory Radio (HAR), Variable Speed Limit signs (VSL), traffic sensors, and Traffic Management Centers (TMC) are systems that control traffic in adverse weather conditions.

The majority of systems that control traffic in adverse road conditions are integrated with other weather-related technologies. For example, many traffic control systems can also disseminate traveler information. Others are designed to disseminate traveler information and detect weather. Still others can disseminate traveler information, detect weather, and forecast weather.

The Rationale for Traffic Control Systems for Adverse Weather Conditions

In December 1990, a heavy fog resulted in a chain-reaction collision involving 90 vehicles on Interstate 75 in southeastern Tennessee. In response to the disaster, a fog detection and warning system was installed along this Interstate in 1994. This system advises motorists of prevailing conditions via flashing beacons atop six static signs, two Highway Advisory Radio (HAR) transmitters, and ten Dynamic Message Signs that reduced speed limits using variable speed limit signs. Since the fog warning system was installed, there have been no fog-related accidents on that highway. This dramatic decrease in weather-related accidents is indicative of the safety capabilities of systems that control traffic in adverse weather.

Adverse weather conditions have a major impact on the operation of US rural and Interstate roads alike. Approximately 7,000 people lose their lives and 450,000 people incur injuries due to adverse weather each year. The accident risk is even higher when there is rain, snowfall, or snow on the road; the risk is 20 times greater when road surfaces are icy. Most drivers should reduce their speed in these conditions but do not, making traffic control measures an essential part of surface transportation safety. Traffic control systems for adverse weather conditions are aimed at decreasing the numbers of weather-related accidents and fatalities.

SYSTEM DESCRIPTION

VMS and Weather

Variable Message Signs (VMS) are the most commonly used traffic control technology. VMS are programmable traffic control devices that display letters and/or symbols. During inclement weather, VMS can display weather information, route guidance, and speed recommendations to drivers. VMS messages range from warnings to recommendations to regulations. Their purpose is to reduce accidents and congestion as well as assist drivers and increase network performance. The main reason that VMS are so effective is that they are highly visible along roads so that most drivers will notice them whether or not they are looking for them.

Intelligent Speed Adaptation (ISA) and Weather

ISA is a term for systems in which the speed of a vehicle is permanently monitored within a certain area. Although still in the testing phase, ISA is an increasingly viable option for enforcing vehicle speed in many road environments, including roads affected by adverse weather. An ISA system used exclusively to control vehicle speeds in certain weather conditions is called a Weather-related Intelligent Speed Adaptation (WISA) system.

There are three main ISA variants:

1. The closed variant: The system intervenes directly with the fuel supply so that it is impossible to exceed the speed limit.

2. The half-open variant: When the speed limit is exceeded, the intelligent gas pedal in the vehicle becomes resistant to pressure so that the driver must use force to push the accelerator.

3. The open variant: When a driver exceeds the speed limit, he or she is informed or warned by a visual or auditory signal, or a combination of both.

ISA is being tested in the Netherlands and Sweden, and one trial has been completed in Tilburg (October 2000). Of these test areas, Sweden is conducting the largest ISA project. It includes 6000 equipped vehicles in 4 municipalities and is expected to cost Euro 9 million. The goal of this program is twofold: to achieve road safety improvements and to achieve driver acceptance of ISA.

Four different ISA systems are being tested in Sweden:

Informative ISA system "Speed checker"
This system includes a small box that is attached to the dashboard. When the driver exceeds the speed limit, a lamp on the box flashes and a sound signal is heard. If the speed increases further, the intensity of the signal increases. Around 5000 vehicles will be equipped with this system, and it is being tested in Umea.

Informative ISA system with display
This system includes a display that shows the current speed limit. The technology used to inform the vehicle of the existing speed limit will be local transmitters, speed signs, and transponders in the vehicle. Around 200 vehicles are equipped with this technology, and it will be tested in Borlange and Lidkoping.

ISA system for quality assurance
This system is used for quality assurance of safe transports. It includes a unit that registers and stores any speed violations. If the driver fails to reduce speed (in 10-15 seconds) in spite of warnings from flashing lamps and sound signals, a registration will be made. This system will be active in Borlange.

Actively supporting ISA system "Active Accelerator"
When the driver drives at the maximum permissible speed, a slight resistance in the accelerator is activated. When the maximum speed is exceeded, an after-mounted servo on the gas wire is activated (otherwise known as an intelligent gas pedal), and the speed is registered via GPS and digital map. The driver can switch the system off by pushing down very powerfully on the accelerator. This system will be active in Lund and Lidkoping.

Each city involved in ISA testing will conduct its own local evaluations based on the following questions: 1) What does the user think? 2) How can the technology be integrated with the driver? 3) What are the effects on traffic and safety?

The Future of ISA/WISA
The expected benefits of ISA include a calmer way of driving, decreased speed limits at intersections, less damage done in vehicle to vehicle collisions (due to lower speeds), and fewer red light violations. If ISA were a mandatory system, it is estimated that it could reduce fatal accidents by as much as 37% (compared to only 18% for an advisory system). ISA is also environmentally friendly on several levels: it is expected to decrease fuel consumption by 11%, Nitrogen Oxides by 11%, and it can act as a traffic calming mechanism without calling for physical road redesign (i.e. no new signs need to be installed). Moreover, the Weather-related Speed Adaptation (WISA) system tested in Finland (see Case Studies) showed that speed control technologies significantly enhance driver safety in adverse road weather conditions.

According to Oliver Carsten, Director of Research at the Institute of Transport Studies at the University of Leeds, making ISA mandatory by 2020 is possible. The main barriers to implementation are the long time need to develop standards for ISA technologies and the need to win public confidence in the system.

Additional Components of Traffic Control Systems

Most other traffic control systems for adverse weather are comprised of several ITS technologies.

• Highway Advisory Radio (HAR) and Variable Message Signs (VMS) are frequently combined to inform drivers of road-weather conditions and to instruct them to take alternate routes.

• Variable Speed Limit (VSL) signs are used in conjunction with VMS and HAR in order to enforce a safe speed limit on roads affected by poor weather conditions.

How are Traffic Control Systems for Adverse Weather Integrated with other ITS Technologies?

Traffic control systems for adverse weather conditions are usually integrated with other weather-related ITS technologies. The result is a variety of traffic control systems that have different capabilities. Here are examples of programs that illustrate these combinations:

Traffic control systems for adverse weather conditions that only include traffic control technologies:

VMS are used in UTAH for foggy conditions. This system relies only upon variable message signs to enforce traffic behavior in adverse weather conditions. The VMS recommend maximum driving speeds for foggy conditions.

Traffic control systems for adverse weather conditions that can disseminate traveler information:

The Weather Related Intelligent Speed Adaptation (WISA) project in Finland combines a speed control system with a traveler information system. WISA technology enforces speed limits through computerized control of vehicle speed when icy road conditions are detected. A VMS located inside the vehicle informs the driver of the impending ice on the road.

Traffic control systems for adverse weather conditions that disseminate traveler information and detect weather:

The Idaho Storm Warning System measures visibility and includes both VMS, which indicate to travelers the current level of visibility, as well as VSL, which enforce speed limits.

Traffic control systems for adverse weather conditions that disseminate traveler information, detect weather, and forecast weather:

The Tennessee Fog Detection and Warning System includes fog detectors, speed detectors, VMS, and VSL. While continually monitoring fog and speed data, a computer system predicts conditions conducive to fog formation. By correlating field sensor data with pre-determined response scenarios, the computer can alert highway personnel of the impending foggy conditions. It also transmits this information to VMS, HAR transmitters, and VSL, all of which can instruct drivers to reduce their speed.

ASSESSMENT

Key Results

• The various combinations of traffic control systems for adverse weather conditions have different degrees of success depending on their accessibility and efficiency. For example, most drivers adhere to VMS weather-related messages because of their high visibility, whereas many people do not listen to HAR traffic reports because they prefer more entertaining radio stations.

• VMS successfully decrease mean speed and standard deviation in poor visibility conditions such as fog.

• ISA/WISA is one of the more successful vehicle control systems because it enforces speed limits through computerized control of vehicles. For example, WISA warns drivers of specific road conditions (i.e. black ice) that require them to adopt lower-than-advertised speed limits.

Benefits

• When VMS or other ITS technologies instruct drivers to change their speed limits or take alternate routes due to adverse weather conditions, there are fewer weather-related accidents.
• VMS and WISA lead to improved driver awareness, reduced speeds, increased headways, and more homogenous traffic flow. All of these factors reduce frequency of accidents.

Costs

• VMS: Capital Cost: 10-50k, Operation and Maintenance: 1.9-4.1k per year
• ISA: estimated cost of equipment is $226-$2265
• HAR: Capital Cost: 16-32k, Operation and Maintenance: 0.6-1k per year
• VSL: Capital Cost: 3.7-5k, Operation and Maintenance: Not provided

Implementation and Operational Challenges

VMS

• Irregular sign spacing is a common problem with VMS.
• Sign comprehension problems (i.e. signs may be physically difficult to read or the message itself ambiguous).
• Drivers may have trouble deciding how to change their behavior.
• Focus on VMS may lead to decreased attention to other driving tasks.
• VMS need to be placed at particular sites with warnings that are specific for each location (versus providing general display messages with area-wide application).
• In the event of an accident, responsibility may be delegated to the system rather than to the drivers.

ISA/WISA

• It will take a long time (20+ years) to fully implement ISA in commercial vehicles.
• Drivers may be resistant to WISA because of longer driving times per route and general dislike of intervention in the driving process.
• Drivers may begin to feel overly confident when using vehicle control systems like WISA and attempt to drive at faster, unsafe speeds.

Other traffic control technologies for adverse weather

• It may take a long time to acquire statistically significant results to prove the efficiency of VMS or VSL.
• Improved connection to HAR is needed when VMS cannot provide enough information to motorists.
• The major challenges to the efficiency of all of these systems are human factors, such as unpredictable driver responses to VMS or VSL.

• Tennessee Fog Detection and Warning System: Tennessee
• Idaho Storm Warning System: Idaho
• Adverse Visibility Information System (ADVISE): Utah
• Fog Warning System: California
• Automated Fog and Smoke Warning System: Georgia
• VMS: throughout the United States
• A 16 Motorway Fog-Signaling System: Netherlands
• ITS-based Traffic Operation Strategy in Poor Visibility Environment: Japan
• Weather Related Intelligent Speed Adaptation (WISA): Finland
• Weather Controlled Road and Investment Calculations: Finland
• Surface Transportation Weather Decision Support Requirements (STWDSR): throughout the United States 

CASE STUDIES

Tennessee Fog Detection and Warning System

In December 1990, a chain-reaction collision involving 99 vehicles prompted the design and implementation of a fog detection and warning system on Interstate 75 in southeastern Tennessee. The system covers 19 miles including a three-mile, fog-prone section above the Hiwassee River and eight-mile sections on each side.

System Description: Center managers with the Tennessee DOT and Tennessee Highway Patrol access a central computer system that collects data from eight fog detectors, and 44 vehicle speed detectors. By continually monitoring fog and speed sensor data, the computer system predicts and detects conditions conducive to fog formation, and alerts managers when established threshold criteria are met. Highway Patrol personnel visually verify onsite conditions.

The computer system provides decision support by correlating field sensor data with pre-determined response scenarios. Operational techniques include advising motorists of prevailing conditions via flashing beacons atop six static signs, two Highway Advisory Radio (HAR) transmitters, and ten Dynamic Message Signs, reducing speed limits using ten VSL signs (i.e., 50 mph or 35 mph), and restricting access to the affected highway section with ramp gates under the worst-case scenario (i.e., visibility less than 240 feet).

Results: There have been over 200 crashes, 130 injuries and 18 fatalities due to fog on this highway section since 1973. Since the installation of the fog detection and warning system in 1994, no fog-related accidents have occurred.

Source: Tennessee Fog Detection and Warning System

Idaho Storm Warning System

The Storm Warning Project was initiated in 1993 as a result of a large number of serious traffic crashes that occurred during periods of low visibility on I-84 in southeastern Idaho between 1988 and 1993. The purpose of the operational test was to investigate various sensor systems that could provide accurate and reliable visibility and weather data, and to use that data to provide general warnings, speed advisories, and possible road closure information to travelers on a section of I-84 in southeast Idaho that is highly prone to reduced visibility from blowing snow and dust. The primary goal of such a system is a major reduction in visibility-related multi-vehicle accidents in rural areas.

System Description: Sensors measuring traffic, visibility, roadway, and weather data were installed at the test site, and automatic traffic counters recorded the lane number, time, speed, and length of each vehicle passing the sensor site. To confirm visibility readings provided by the sensors, a video camera was installed at the test site and aimed at a series of target signs placed along the interstate at various known distances. During the course of the project, four variable message signs (VMS) were installed along the test section of roadway, to provide information to travelers regarding low visibility and other road condition information in the test area. Data generated by the sensor systems was transmitted to a master computer, which recorded readings every five minutes. This information provided a baseline of driver behavior, to help determine if the signs were causing drivers to change their behavior. Information was transmitted to the motorist via changeable message signs. Cost: Estimated Total ITS Funds: $804,500 Estimated Total Project Cost: $1,231,900.

Source: Idaho Storm Warning System Operational Test, Final Report, Idaho Transportation Department, December 2000.

Oklahoma's First-response Information Resource System using Telecommunications (OK-FIRST) Project

System Description: OK-FIRST is an initiative by the Oklahoma Climatological Survey to improve access to current weather information and to develop a decision-support system for the state's public safety (fire, police and emergency management) agencies.

OK-FIRST transmits current weather information to emergency managers in various public safety agencies. To aid in effective decision-making, the system provides tailored county-level, agency-specific weather information that allows emergency managers to accurately identify weather threats. OK-FIRST includes a web site and an electronic bulletin board system to foster communication and facilitate information sharing. Emergency managers have used OK-FIRST surveillance data to respond to severe weather (e.g., floods and fires) and HAZMAT incidents; to execute control (e.g., close bridges and interstate highways) in coordination with traffic managers; and to plan response (e.g., staff snow crews) in coordination with maintenance managers.

Source: OK-FIRST

Utah DOT Adverse Visibility Information System (ADVISE)

System Description: ADVISE is a speed advisory system that the Utah Traffic Lab (UTL) and the Utah Department of Transportation (UDOT) installed near the highway to evaluate visibility and recommend speeds to drivers. ADVISE continuously measures visibility and displays the safe speed when visibility descends below a fixed threshold.

Results: UTL and UDOT have measured and evaluated the effectiveness of ADVISE on the I-215 corridor in Salt Lake City, Utah, where fog is a recurring problem. This system resulted in reduced variability of traffic speeds during low visibility events while the average speed increased. This occurred because overly cautious drivers were advised to travel at speeds higher than they initially thought. In this regard, ADVISE reduced the difference between the fast drivers and the slower drivers by 22% (1997-1998) during lower visibility conditions.

Source: ADVISE

The Fog Warning System in California

The Central Valley can experience extremely heavy fog during the winter months. Caltrans and the CHP began "Operation Fog" in 1991 in an effort to help make drivers aware of this hazard in the Central Valley and to reduce the number of fog-related accidents.

System Description: Caltrans established an automated "Fog" warning system in 1996 in San Joaquin County on southbound Interstate 5 and westbound SR 120, areas known for dense fog. The automated system consists of nine permanent Changeable Message Signs (CMS), nine weather stations, and thirty-six speed monitor locations. The Fog Warning System automatically detects reduced visibility and speeds. The system automatically advises travelers, via the CMS, of speeds that would be safe for conditions ahead. When visibility is 0' to 100' the message "Dense Fog Ahead" will appear to alert the motorist of conditions ahead. When visibility drops to 201' to 500' – "Foggy conditions Ahead" would appear. When traffic speeds are 11 - 35 mph the message "Caution Slow Traffic Ahead" will be shown. When vehicle traffic speeds drop between 1 – 10 mph the message "Caution Stopped Traffic ahead" will appear on the CMS. In the event of an accident during foggy conditions the speed messages will override the fog messages.

Results: The Caltrans Fog Warning System has thus far been shown to deter vehicle accidents that usually result from thick fog or dust storms along I-5 and SR-120 in San Joaquin County.

Source: Caltrans Article

Automated Fog and Smoke Warning System in Georgia

A heavily traveled, 14-mile stretch of interstate highway in south Georgia cuts through a peat bog, which often produces extremely thick fog. Vehicle accidents and human fatalities have resulted due to the low visibility. Engineers at the Georgia Tech Research Institute (GTRI) in Atlanta proposed a solution, and in the summer of 2001, a $2.4 million, automated fog and smoke warning system will become operational on Interstate 75 near Adel, Ga., just 35 miles north of the Florida state line.

System Description: At the heart of the system's data collection operation is a set of 19 commercially available fog sensors. Each sensor has a transmitter and a receiver. The transmitter is angled away from the receiver, so that in clear conditions, its light beam misses the receiver. But when fog or smoke particles are present in the air, they scatter some of the light into the receiver. The sensor detects this light and sends a reading to the system's on-site computer. The computer, linked to the system's components via a fiber optic network, is located inside a building alongside the interstate in the fog zone. The computer collects data not only from the fog sensors, but also from an adjacent weather station, speed detectors on the highway and television cameras, which GDOT officials can operate remotely to scan the area. Also, the computer contains several telephone modem lines so officials can remotely upload and download data from the site.

The system's software analyzes data from the fog sensors, notifies GDOT officials at the Transportation Management Center (TMC) in Atlanta of potential problems and automatically decides what message to post on four changeable message signs on the north- and southbound outskirts of the fog zone. The 36-foot-wide and 9-foot-high signs are attached to metal structures built over the highway.

When the fog warning system detects a decline in visibility to a certain level, it automatically notifies the TMC and also sends a caution to motorists via the changeable message signs. It also turns on streetlights along the roadway. If visibility drops to a second threshold, the system again notifies the TMC and then issues a speed advisory to motorists via the signs. The system recognizes two additional lower levels of visibility and again alerts the TMC and issues advisories to motorists to further decrease their speed. TMC personnel plan to develop a protocol for notifying local officials, who could put GDOT people on the scene at some point as the visibility gets worse.

Results: The GTRI will conduct a follow-up on the warning system after it becomes operational. Researchers will pay special attention to the system’s maintenance requirements and accuracy as well as to human factors such as driver response to CMS.

Source: Georgia Tech Research News Article

VMS in UTAH used for foggy conditions

System Description: Visibility sensors and variable message signs (VMS) displaying recommended maximum speeds in Utah's Salt Lake Valley were implemented to reduce the variability of driver's speeds under foggy conditions.

Results: Comparison of vehicle speed data collected before and after the activation of the VMS visibility warning system indicated that mean vehicle speeds increased after the system was activated. However, the standard deviation of vehicle speeds within the affected freeway segment decreased significantly. It is notable that this segment of Interstate was widened from 3 to 4 lanes between the before and after study periods. This makes it difficult to determine whether the impact on vehicle speeds was a result of the ITS implementation or the roadway widening project.

Prior to activation of the signs, the average vehicle speed was 54 mph with a standard deviation of 9.5 mph. After the system began operation, average speeds increased to 62 mph, with a standard deviation of 7.4 mph. This represents a 15% increase in speeds and a 22% decrease in the standard deviation of those speeds.

Source: ITS Benefits Database (Measurement: Safety)

A16 Motorway Fog-Signaling System (Netherlands)

System Description: On the A16 Motorway in the Netherlands, an automatic fog-signaling system was implemented October 1991 to elicit safer driving behavior during fog. The system uses 20 sensors along the 12 km stretch to measure visibility. Based upon the visibility distance calculated, a certain speed limit is shown on overhead message signs.

Results: The system has a positive effect on speed choice in fog: it was found to result in an additional decrease of speed of about 8 to 10 kph and a slight reduction in standard deviation of the speed. In extremely low visibility, the system had an adverse effect. The average speed with the system in this situation was 60 kph, versus 29 kph without. Using the relation between mean speed and number of accidents, a reduction of 5 kph would result in a similar reduction in the number of accidents by approximately 15%. The system showed small or no effects in other measures of driving behavior such as following distance, time headway, and time to collision.

Source: ITS Benefits Database (Measurement: Safety)

ITS-based Traffic Operation Strategy in Poor Visibility Environment on Inter-Urban Expressways (Japan)

System Description: The system used administrative pace-vehicles equipped with Millimeter Radio Wave Sensors and GPS technology to lead freeway traffic through heavily fogged areas subject to road closures. The system would attach sensors to leading-vehicles and allow groups of freeway traffic to follow using a warning vehicle in the rear. The Emergency Management center would monitor each ITS-vehicle using GPS and enable them to track each other’s position. Guidance vehicles were scheduled to provide service every 15 minutes on the Kanetsu Expressway, and every 20 minutes on the Oita Expressway.

Results: The results showed that traffic control using guidance-vehicles was cost effective. The overall benefit-cost ratios ranged from 1.7 to 2.1 for the Kanetsu Expressway. The overall benefit-cost ratios ranged from 1.8 to 2.2 for the Oita Expressway. The Millimeter Radio Wave Sensor performed well under foggy conditions. However, its performance was greatly influenced by the size and shape of objects as well as by the waves reflected from adjacent obstacles.

Source: ITS Benefits Database (Measurement: Safety)

Weather-related Intelligent Speed Adaptation (Finland)

System Description: The Technical Research Centre of Finland conducted a two-phased driving simulator study to compare adverse road condition driver support systems and to test a method of informing drivers of adverse road conditions. The study was performed using the advanced driving simulator of the Institute for Transport Studies at the University of Leeds, UK. This fixed-base simulator is a complete vehicle with all basic controls, operational dashboard instruments, and driver feedback provided via simulated steering forces in the steering wheel. The test route designed for the study consisted of a rural road with a 50 mph speed limit.

In phase one, 16 drivers drove the test route three times: unassisted in summer conditions, unassisted in winter conditions, and with a risk display in winter conditions. The risk display informed drivers about road surface friction with a warning bar that grew longer and changed color with increasing risk. In phase two, 24 drivers were used to compare the following driver support systems under winter conditions: no support system (i.e., typical winter driving feedback), an advanced driver information system, and the Weather-related Intelligent Speed Adaptation (WISA) system. The advanced driver information system included variable message signs indicating "ICE" if road surface friction was low. The WISA system prevented vehicles from exceeding a safe speed on icy road sections.

Results: In the first phase, average travel speeds in icy winter conditions were 2.5 mph lower than summer travel speeds, 39.0 mph (62.8 km/h). Although the risk display decreased speeds in sharp curves, test drivers suggested a more anticipatory system, which would warn drivers before they encountered a curve or icy area. In this phase, only one driver ran off the road on a sharp icy curve. In phase two, the coverage of low-friction areas was increased and the friction of non-icy sections was lowered to better correspond to normal winter conditions. Five drivers with no support system ran off icy road sections and five drivers ran off the road when using advanced driver information system. No test drivers assisted by the WISA system left the roadway.

Conclusions: It was concluded that driver adaptation to adverse conditions is not adequate because they cannot accurately assess the degree of road surface friction. The Weather-related Intelligent Speed Adaptation (WISA) system appeared to support drivers in adverse road conditions, increasing safety and travel speeds. More advanced measurement techniques are required to provide drivers with adequate road condition information and to recommend appropriate behavior modification.

Source: ITS Benefits Database (Measurement: Safety)

Weather Controlled Road and Investment Calculations (Finland)

System Description: The 8.7 mile test area is equipped with a road weather information system (RWIS) consisting of 36 variable speed limit signs, five variable message signs (VMS) displaying graphical and textual information, and two environmental sensor stations (ESS). Each ESS measures wind speed and direction, air temperature, pavement and sub-surface temperature, humidity, precipitation rate and accumulation, and pavement condition. The western ESS nearest the sea also measures precipitation type and visibility. Variable signs are automatically controlled by the RWIS or manually controlled by operations personnel. Manual control is used if the variable signs automatically display information that does not correspond to actual conditions. Variable speed limit signs are divided into 12 separately controlled groups, with signs in a single group displaying the same speed limit.

Results: The divided roadway section typically has speed limits of 75 mph in the summer. In the winter, recommended speed limits vary between 50 mph and 62 mph based on road and weather condition data collected from the ESS. Recommended speed limits are based on pavement condition, precipitation, visibility and wind. Heavy precipitation, wet pavement conditions, reduced visibility, and wind speed all reduce the speed limit. When speed limits are reduced, VMS display the reason for the reduced speeds. Three symbols indicating "slippery road surface," "hazardous conditions ahead," or "road construction ahead" may also be displayed on VMS. If speed limits are not reduced, VMS display only air and pavement temperatures.

Conclusions: It was estimated that the average speed decreased 0.4 to 1.4 percent due to the RWIS. The average yearly accident rate was projected to decrease by eight to 25 percent. This impact on safety is due to lower average speeds when accident risk is greatest (i.e., when poor road conditions exist).

Source: ITS Benefits Database (Measurement: Safety)

REFERENCES

Crawford, David. A WISA way to drive: intelligent speed adaptation (ISA) can have a positive role in automatically implementing weather warnings, believe Finnish traffic researchers. ITS International. 2000.

Gunnar, Lind. Large-scale testing of intelligent speed adaptation: important issues. European Transport Conference (1999: Cambridge England), pp. 91-102. PATH record no. 19718. Abstract available in the PATH database.

Pedic, Fadil. A literature review: the content characteristics of effective VMS. Road and Transport Research, pp. 3-11, 1999. PATH record no. 17620. Abstract available in the PATH database.

Rama, Pirkko. Effects of weather controlled variable speed limits and warnings signs on driver behavior. National Research Council, p. 12, 1999. PATH record no. 16072. Abstract available in the PATH database.

Rama, Pirkko. Effects of variable message signs for slippery road conditions on driving speed and headways. Transportation Research, pp. 85-94, 2000. PATH record no. 20727. Abstract available in the PATH database.

Author: Lauren Smith, last update: 11/01/01