Road Management & Engineering Journal
Road Management & Engineering Journal
January 1, 1998
TranSafety, Inc.
(360) 683-6276
Fax: (360) 335-6402

National Cooperative Highway Research Program (NCHRP)
Synthesis of Highway Practice 228:
Reduced Visibility Due to Fog on the Highway, A Synthesis of Highway Practice

(The following information is from "NCHRP Synthesis of Highway Practice 228: Reduced Visibility Due to Fog on the Highway." Reproduced here is "Summary" and "Chapter Six: Conclusions." These sections outline the contents of the publication. A copy of the complete document can be purchased from TranSafety, Inc. (57 pages, $20 plus $4 shipping and handling).)


In the United States during 1990 and 1991, four multiple-vehicle accidents caused by reduced visibility conditions resulted in 21 fatalities. Such catastrophic accidents dramatize the hazard of reduced visibility on the highway caused by fog, dust, or smoke. The extreme variability in density, predictability, and location of the hazard further complicates the task of improving highway safety conditions.

Much of the problem stems from inadequate traffic control techniques to provide specific behavior guidance for drivers in areas of reduced visibility, and from the unpredictability of when and where those techniques are needed. Also of great concern [are] erratic driver behaviors, including excessive or variable speeds, following too closely, or stopping on the roadway, which increase potential for accidents. Once an accident occurs, reduced visibility increases the likelihood of secondary accidents, where vehicles collide with those already involved.

Because of the ongoing problems associated with reduced visibility, emphasis on countermeasures has increased in the United States and Europe. This synthesis describes countermeasures implemented in California, Louisiana, New Jersey, New Mexico, South Carolina, and Utah, and in England and the Netherlands.

In California, significant initiatives have been taken to address the persistent problems of fog and dust in the Central San Joaquin Valley. Efforts in public awareness have been well received as witnessed by favorable public perceptions to fog countermeasures and an increase in the number of radio and TV stations seeking information. California Highway Patrol [CHP] units used to pace traffic and to provide an enforcement presence were also believed to be effective in reducing speed and accidents. Similar projects are planned for Stockton and San Bernardino.

Louisiana has implemented countermeasures for fog on the Lake Pontchartrain Bridge for some time and recent improvements, including the use of changeable message signs (CMSs) and the restricting of vehicles to one lane have been successful. The system of speed limit and warning signs on the New Jersey Turnpike which alert motorists to driving conditions ahead is considered to significantly reduce secondary accidents. The variable speed limit system installed in New Mexico was found to respond quickly to changes in traffic conditions; however, driver behavior did not appear to change significantly after specific message signs (WET AHEAD, SLOW AHEAD, WRECK AHEAD) were displayed.

South Carolina installed a traffic management system on the Cooper River Bridge that includes active and passive traffic control features, in addition to weather detection and surveillance capabilities. Tennessee has installed a fog detection/warning system consisting of CMSs with vehicle flow detectors for monitoring speed.

Utah has researched the use of aerial and ground seeding techniques for the dispersion of fog. These experiments have been successful in dispersing fog to improve safety; however, the success depends on logistical planning along with meteorological and physical conditions that are within the functional limits of the seeding concept.

European initiatives involved fog detection and monitoring on M25 circling London, where fog signs and CMSs were being used. The Dutch have implemented countermeasures that include traffic flow detectors, fog detection, automated traffic control, and enforcement. There is a substantial coordinated effort in the European community toward safety associated with reduced visibility situations.

In 1991, the National Transportation Safety Board [NTSB] convened a special hearing on fog accidents on limited-access highways. Numerous suggestions, conclusions, and recommendations for countermeasures resulted from the hearing and these are summarized in Chapters 6 and 7.

Research and development continued with initiatives being undertaken in various states along with those being supported by the Federal Highway Administration. There does seem to be an urgency to implement the Intelligent Vehicle Highway System capabilities because of the real potential for improvements in vehicle-driver communications.

This report cites a number of ongoing and planned studies; specifically, California, Georgia, Idaho, New Mexico, South Carolina, Tennessee, and Utah, in addition to those abroad. Because of the time factor in publishing this synthesis, additional information may now be available from those and other sources.


The synthesis examines the reduced visibility countermeasures that have been developed in the areas of fog dissipation, traffic control, and public awareness. Some experimental results indicate that improvement in road conditions can be demonstrated to disperse or reduce fog and to improve safety and serviceability through aerial or ground seeding. However, more investigation is needed to demonstrate effective and reliable tools to dissipate fog under a variety of climatological, geographic, and road conditions.

Programs to reduce fog related collisions in California have demonstrated that morning broadcasts on radio and TV stations during periods of heavy fog and the distribution of fog safety brochures were successful in increasing public awareness. Other means of informing the public about real conditions are the use of bulb matrix displays of the portable CMSs, which are visible even during dense fog; the use of portable HARs [highway advisory radios], which make additional information available to motorists; and the use of CHP PACE [special enforcement] units. Increased law enforcement presence is thought to be a major factor in improving driver diligence in a heavy-fog situation.

When causeway driving is a factor in a fog environment (as it is in Louisiana), CMSs, HARs, and pamphlets are used along with traffic control measures that allow traffic to move in the right lane only (2 lanes in each direction), with police riding herd to prevent passing and to look for possible breakdowns. Closure of the left lane is a key to maintaining smooth traffic flow as it allows a place for motorists to pull into in the event of a problem.

Secondary accidents are a major problem under driving conditions of reduced visibility. A system of speed limit and warning signs was used in New Jersey and New Mexico and was thought to significantly contribute to the reduction of secondary accidents by alerting motorists to driving conditions ahead. However, traffic behavior, as defined by average speed and standard deviation in speed, does not appear to change significantly after signs are displayed.

The states have also been guided in their efforts by recommendations of the National Transportation Safety Board. An NTSB special public hearing on fog accidents on limited-access highways provided the following suggestions and conclusions:

There is a major focus on further developing reduced-visibility countermeasures, as witnessed by ongoing and planned projects in at least eight states (in addition to efforts abroad). It is very important that this new information be used to advance the knowledge surrounding the effectiveness of reduced-visibility countermeasures. It is recommended that the following areas be considered.

Motorist Information Systems--It is important that the results of ongoing and planned projects involving motorist information systems be analyzed, in combination with those already available, to better understand how information on reduced-visibility conditions influences the driver. This would help establish guidelines for installing systems that are effective and that offer uniform and credible information to the driver. Special consideration should be given to the following:

Consideration should be given to studies involving driver behavior relative to the psychophysics of driving under conditions of reduced visibility to help understand how drivers react and what the psychological elements of depth perception are. How the brain interprets the eye's visual cues in the distance-judgment process and whether faulty judgments are made as a result of misinterpretation are matters of concern. A better understanding of this would help in determining what visual information is best to give to motorists.

Other Traffic Control Techniques--Traffic control techniques that have been effective in specific locations or situations may be useful in other areas or for corroborating the technique's effectiveness. A good example is the PACE program in California and the single-lane concept on the Lake Ponchartrain Bridge in Louisiana.

Fog Detection--There continues to be a need for reliable, economical methods of detecting conditions of reduced visibility; information from ongoing and planned projects on detection will be useful.

Education and Public Awareness--Educating the public concerning the hazards of reduced visibility and the problems associated with driving is an important step in increasing driver safety in these conditions. California's success is a good example of the benefits that result from better public awareness.

Fog Dissipation--Considering the success of fog dissipation experiments using seeding techniques, these may be investigated further so that guidelines for their use on the ground and in the air can be established.

IVHS--Solutions to the hazards of driving during reduced-visibility conditions using IVHS technology hold promise for effective mitigation.

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