Is it safe to bike commute in foggy weather?

Fog is a fact of life in the San Francisco Bay Area, and many other parts of the world. Some days it's really just a low overcast cloud layer that mutes the landscape but does little to deter our thoughts of riding. Other times it can be thick a pea soup. How safe is it to ride in such conditions? Let's break it down into a few factors:

Fog, but not a safety factor given there is a protected
bike lane and still plenty of visibility
How thick is the fog?
Not all fog is created equal. Once forward visibility gets below 50m / 160ft it starts to really affect traffic. In the SF Bay Area, it can get scary thick, down to 10m or so, especially in places like Daly City at the Skyline / Highway 1 interchange and on the way up to Mount Tam.

Do you have lights?
The most obnoxious, bright blinking lights are your friend when the fog rolls in. I wouldn't ride in heavy fog without something (or multiple things) on the rear, and probably also on the front if I am on a bike path where head on bicycle or pedestrian traffic is likely.

Is there a protected bike lane or wide shoulder?
If you have lots of safe space to ride, then even the thickest fog shouldn't be too much of a problem on flats and uphills. You might have to brake on descents so that you can maintain a safe amount of visibility in front of you.

How fast is the car traffic going?
Cars will slow down when the fog gets bad, but often they won't slow enough as much as they should. If you have cars going highway speeds (or anywhere near that fast) then their braking distance can easily outrun their visibility in thick fog. For example at 80kph / 50mph cars will need 53m / 174ft of stopping distance (according to this site). When roads are straight and drivers are used to driving fast on a stretch it's common to see cars going way too fast for the conditions. If the road is super windy then prospect of driving off the edge usually slows drivers down significantly when visibility is poor.

This fog on Mt Tamalpais was almost comically thick. I chose
to ride on since traffic was light and the twisty road
would keep drivers from going too fast
So let's look at a couple scenarios. If the fog is crazy thick, you don't have lights, the shoulder is narrow or nonexistent, and the traffic blazing fast then it's an easy answer: no-go. If in the same weather conditions you can stay separated from traffic and have some good lights then you can probably proceed safely. Use your judgement and don't be afraid to bail out.

One other note: fog = moisture = slick surfaces. Be prepared for reduced braking effectiveness on the bike (especially for rim brakes) and slippery spots on the road (especially metal plates and those bumpy plastic crosswalk ramp surfaces)


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