Though fog is an isolated and somewhat regional and seasonal road hazard, it is particularly challenging for drivers. A 2014 report by the AAA Foundation for Traffic Safety, looking at federal crash data on fatal crashes from 1990 to 2012 and police-reported crashes from 1990 to 2008, found that fog was a factor in nearly 20 percent of deadly multicar pileups involving 10 or more vehicles.
Fog is especially prevalent in some regions, including much of the Southeast, northern New England, the Pacific Northwest and the Central Valley of California, and it forms most often in winter. Deadly multicar crashes generally occur when cars and trucks traveling at interstate speed drive into what is essentially a low-lying cloud and quickly lose visibility. Drivers may not see the slowed cars ahead until it is too late, with one vehicle crashing into the next, including huge tractor-trailers.
Eleven people died in a pileup in January 2012 near Gainesville, Fla., a crash that was linked to a combination of smoke and fog. In November 2007, a chain-reaction crash of 108 vehicles in fog near Fresno, Calif., resulted in two deaths, as did a 60-car pileup in Wyoming in April 2015. On Jan. 31, nearly 50 cars piled up in fog-related crashes near Hanford, Calif.
Lighting and safety experts caution that stand-alone front fog lights, which are usually set into the bumper close to the road, may be inadequate to prevent such horrific highway-speed crashes. But they can help drivers see road markings in fog at low speeds, perhaps keeping the car from hitting a tree or running into a ditch.
High-beam headlights, designed to send light into the distance, are especially ineffective at penetrating the fog, as they reflect off the moisture in the air. But even low beams throw enough light into the fog curtain that the effect can be blinding rather than illuminating.
Fog lamps are intended to provide an adjunct to the low beams. Because fog hovers close to the ground, the lamps are designed to shine down, illuminating the road beneath the fog. The top of the beam is cut off sharply so the light does not shine into the fog and reflect off it.
Jennifer Stockburger, the director of operations at the auto test center of Consumer Reports, said that although her magazine routinely tests headlights as part of its auto evaluations, it does not concern itself with fog lamps, which are “just meant for that low light in front of the bumper.”
She added: “They do work because they cut under the fog. But they are so short in range that you’ve got to be going darn slow to make any use of that.”
Of greater concern, she said, is the effectiveness of low beams, adding, “That is how most people are driving most of the time.”
Even before automakers began offering high-tech headlights, many vehicles on American roads did not have front fog lamps. Though common on cars from European manufacturers and on luxury models from American and Japanese companies, they tended to be options on mainstream models, sometimes included on premium trim lines or in extra-cost feature packages.
Aside from the fog lights in the front, which can help the driver to navigate, rear fog lamps — which look like an extra-bright taillamp on just one side — make cars more visible in bad weather to following drivers. Rear fog lamps are required in Europe but not in the United States, though most European manufacturers include them on cars sold in the United States.
And drivers sometimes leave the rear units on even when the skies are clear, just as they do with the front units, which other motorists can find distracting.
While some companies are phasing out discrete fog lights at the front of the car, others are keeping them. Except for a few specialized models, BMW continues to offer front fog lights on models sold in the United States, according to a company spokesman, Hector Arellano-Belloc. And Mark Gillies, a spokesman for Volkswagen of America, said: “No, we are not phasing them out at all. They can be quite useful.”
Others say the latest headlight technology makes separate fog lights redundant.
Michael Larsen, the technical fellow for exterior lighting at General Motors, said the high-tech headlamps on the Cadillac CT6 luxury sedan produce “such an abundance of light” that they not only meet the company’s criteria for factors such as distance, glare and uniform illumination, but also “fill in the foreground of the beam pattern to basically do what fog lights do, creating generous amounts of foreground light.”
“So if there is fog or rain or whatever, you are able to see those road markings,” Mr. Larsen said. Indeed, he added, if fog lamps were fitted to a car like the CT6, “you wouldn’t be able to see that anything changed in the beam pattern, because the fog lights basically cover the same areas.”
Some manufacturers like Mercedes-Benz have used the space once occupied by fog lights for cornering lamps or daytime running lights. Audi has used the niches to install radar units for driver-assistance technology like adaptive cruise control.
Ms. Stockburger of Consumer Reports said that new technologies held out the promise of overcoming fog. She noted that infrared night-vision systems, already offered on some models from companies including Audi, BMW, Cadillac and Mercedes, essentially “see” through fog, snow or darkness by sensing the heat of objects and displaying their images on a screen. Existing technology like radar and lidar, as well as automatic emergency braking systems that stop your car before it hits something, have the potential “to see what your eyes just can’t,” Ms. Stockburger said.
“Maybe the car will come to a stop and you won’t know why,” she said. And with further development of vehicle-to-vehicle communication systems, “your car will then tell the car behind you to stop.”
Not only will fog lights be unnecessary, she added, but “maybe there won’t be headlights at all.”