By Bill Corliss
Owner, Corliss Driving and Traffic School

At a recent “Young Drivers” night at Golden West High School, a mother expressed concern about how to know when it is safe to let her son drive in the fog. At Corliss Driving School, we are frequently asked if we cancel driving lessons when it is foggy. The reality of living in the San Joaquin Valley is that the winter brings fog so we all must learn how to drive safely during foggy conditions.

Although drivers must pay attention to many things when driving in the fog, including not following too closely and selecting safer routes to a destination, the most important consideration is figuring out the appropriate driving speed. In my years of teaching driving, I have learned that even many mature drivers don’t have a process for choosing a safe speed while driving in the fog or other conditions of impaired visibility.

Safety in Numbers
Common sense tells us to slow down in the fog, but what number on the speedometer is safe? Crashes that make the headlines are not caused by fog, they are caused by drivers going so fast that the vehicle doesn’t have enough distance to stop safely. Drivers are estimating safe speeds incorrectly.

“If visibility is 100 feet, then drivers can safely drive 30 mph.” This information is buried in the 2008 DMV Handbook in a section titled “Driving in Rain or Snow!” Drivers can estimate distances in the fog using houses in a residential area or power poles in rural or highway areas. Two average houses are about 100 feet long and power poles are slightly more that 100 feet apart. So, if drivers can see two houses, 30 mph or less is a safe speed. In the country, if your vehicle is next to a power pole and you can see the next power pole, a safe speed is also 30 mph or less.

If visibility is 300 feet, then drivers can safely drive 60 mph. On our freeways (65 mph speed limits) or rural two-lane roads (55 mph speed maximum), drivers must be able to see three power poles or six houses (300 feet) to drive safely in the fog. Automobile testing has shown that we need three seconds to stop a car at 60 mph. At 60 mph, a car will travel 88 feet per second, so in three seconds a car will travel 264 feet, which is just less than the 300 feet of visibility.

Keep it Simple
It is simple. A safe speed with 100 feet of visibility is 30 mph and a safe speed with 300 feet visibility is 60 mph. On freeways, in areas with many fog related crashes, Fog Visibility Checks help drivers estimate the visibility. The first sign displays a zero, then 100, 200, and 300 so that the farthest sign you can see tells you how many feet of visibility exist. Then you apply the same rules: If you can only see the 100, then 30 mph or less is safe, if you can see the 300, then the safe speed would be 60 mph. A few years ago, 400 and 500 feet signs were added for semi-trucks, which take nearly twice as long to stop at 55 mph as cars.

I have taught this theory to thousands of novice drivers in the last twenty-eight years. It is simple and it is safe. We cannot tell our young drivers just to slow down “a little” for mild fog and slow down “a lot” for thick fog, as there are not any “a little” or “a lot” stickers on our speedometers and their safety is too important to leave to guesswork. Keep it simple: 30 mph at 100 feet; 60 mph at 300 feet.