Headlights in the Rain, at Dusk, and in Fog: The California Rule

What the law says: Vehicle Code 24400

Section 24400 has two duties, one about the car and one about the driver:

• Equipment, subdivision (a). A motor vehicle other than a motorcycle must have at least two headlamps, at least one on each side of the front, mounted not more than 54 inches nor less than 22 inches high.
• Use, subdivision (b). The vehicle "shall be operated during darkness, or inclement weather, or both, with at least two lighted headlamps."

Subdivision (c) defines inclement weather as either a condition that prevents a driver from clearly discerning a person or another vehicle on the highway at 1,000 feet, or a condition "requiring the windshield wipers to be in continuous use due to rain, mist, snow, fog, or other precipitation or atmospheric moisture."

What counts as darkness

"Darkness" has its own definition in section 280: any time from one-half hour after sunset to one-half hour before sunrise, and any other time when visibility is not enough to make a person or vehicle clearly discernible at 1,000 feet. That second clause covers smoke from a fire, a dust storm, a tunnel, or a heavy marine layer at midday.

What it means in practice

The wiper rule is the easiest test to apply: if the wipers need to stay on, the headlamps go on. The 1,000-foot test is harder because drivers overestimate how far they can see in gray light. Ask whether a dark car with no lights a fifth of a mile ahead would be obvious. If not, you are in the statute's definition.

A modern trap is daytime running lights. On many cars the running lights brighten the front of the car while the tail lamps stay off, so the dashboard looks lit and the car is nearly invisible from behind in rain. Setting the headlamp switch to automatic, or simply to on, avoids it.

Insurance enters after a collision. Whether your lights were on at dusk, in fog or in rain is one of the first facts an adjuster asks about, because being hard to see is part of how fault is weighed. A car with its lights off in a condition that required them gives the other side an easy argument.

What to do

1. Set the headlamp switch to automatic or on as a habit, not off.
2. When the wipers go on and stay on, check that the headlamps and tail lamps are actually lit.
3. In smoke, fog or a marine layer, use low beams; high beams scatter back into your eyes.
4. Walk around the car once a month with the lights on and replace any dead bulb promptly.
5. After a crash in poor light, note and photograph which lights were on, on both vehicles.

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