Tule Fog and the Yolo Causeway: Driving It Safely

Anyone who has driven the Yolo Causeway on a still December morning knows the feeling: clear sky in Davis, and by the middle of the crossing you cannot see the car in front. Tule fog is the most genuinely dangerous driving condition in this part of California, and it catches people out every winter because it forms so quickly.

The short version

Low beams, not high. Slow down before you need to. Increase your following distance well beyond what feels necessary. And do not rely on your driver assistance systems, because fog is exactly the condition they handle worst.

Why it forms here

Tule fog needs three things the Valley supplies reliably in winter: moisture in the ground after rain, a clear night that lets heat radiate away, and calm air with nothing to mix it. The result is a dense layer sitting close to the ground, often only a few hundred feet deep, which is why you can see blue sky above while unable to see the road ahead.

Low ground near water is where it settles first. The causeway crosses a floodplain, which is precisely the geography that produces it.

What to actually do

• Low beams only. High beams scatter off the droplets and reflect straight back at you. It feels like it should help and it does the opposite.
• Slow down before the visibility drops. Fog banks have edges. If you can see one coming, reduce speed while you still have sight lines rather than after.
• Open the following distance a long way. The usual advice about seconds of gap assumes you can see. Assume you cannot.
• Use the defroster properly. Fog outside means condensation inside. Air conditioning on with the defroster clears glass faster than heat alone, and a clean cabin filter makes a real difference to how quickly it happens.
• Do not stop in a traffic lane. If you must pull over, get completely off the road and turn your lights off so nobody follows your tail lights into the shoulder.

Driver assistance systems are not the answer here

This is worth being blunt about. AcuraWatch and equivalent systems on every brand rely on cameras and radar. Dense fog degrades both. Adaptive cruise control may fail to see a slowing vehicle, lane keeping may lose the markings, and automatic emergency braking may not have enough visibility to act in time.

Most systems will warn you and disengage. Some will simply become less reliable without saying so. Either way, fog is the condition in which you drive the car yourself.

What is worth checking before winter

Nothing exotic, but the items that matter in fog are the ones people neglect:

• Wipers and washer fluid. Fog leaves a film on the glass that needs clearing constantly.
• All exterior lights. Being seen matters as much as seeing. A failed tail light is a serious problem in these conditions.
• The cabin air filter and defroster. A blocked filter slows demisting noticeably. More on why filters load up quickly out here.
• Tires. Fog usually means a damp road surface as well as poor visibility.

If you are heading into the mountains rather than across the Valley, the conditions are different and so is the advice — see our guide to AWD, snow tires and chain controls.

Getting it looked at

A pre-winter check covers all of the above in one visit. For owners in Davis, Woodland and West Sacramento there is no Acura service department on your side of the river, so it is worth checking for open recalls at the same time and making one trip do the work. Book online.

Questions we get asked

What is tule fog?

A dense, low-lying ground fog that forms in the Central Valley in the colder months, typically after rain followed by clear calm nights. Visibility can drop to a few car lengths with very little warning.

Do driver assistance systems work in heavy fog?

Not reliably. Camera and radar based systems including adaptive cruise and lane keeping are degraded by dense fog and may disengage. Treat them as unavailable and drive accordingly.

Should I use high beams in fog?

No. High beams reflect off the water droplets and make visibility worse. Use low beams, and fog lights if the vehicle has them.

Is the Yolo Causeway worse than other roads for fog?

Low-lying areas near water are classic tule fog locations, and the causeway crosses a floodplain. It is a stretch worth treating with particular caution on a cold, still winter morning.

General safety guidance only. Driver assistance systems have limitations and are not a substitute for attentive driving. Always drive according to conditions and follow posted advisories.