Fog: why fog lamps go on earlier, and later, than you think

In late autumn fog turns up where you are not expecting it: in the hollows by the Daugava, on the roads out of the city before dawn, over the fields just past the ring road.

It is also the only weather in which a driver's normal instincts work backwards. Add light and you see less. Mark yourself more brightly and the driver behind stops being able to tell whether you are braking.

"Insufficient visibility" means three hundred metres

The real misunderstanding hides in the term itself: most drivers reach for the fog lamps when they can barely see anything at all. The Latvian Road Traffic Law (Ceļu satiksmes likums) draws the line somewhere else — insufficient visibility means weather conditions (fog, rain, snow and the like) in which visibility on the road is closer than three hundred metres. Twilight counts as well.

Three hundred metres is a great deal of road: you are inside that definition long before the fog becomes frightening. And the definition lives in the Law — the traffic rules themselves (Ceļu satiksmes noteikumi, CSN — Latvia's road traffic regulations, Cabinet Regulation Nr. 279) use the term constantly and never explain it.

Permitted and useful are not the same thing, though: in clear twilight fog lamps are formally allowed, but there is nothing in the air for their light to scatter off.

Front fog lamps: what is actually allowed

First, what not to do: main beam catches the most droplets and sends a good share of the light back into your own eyes, so a glowing wall stands where the road should be. In fog you turn light down, not up (how main beam works the rest of the time).

The rules allow front fog lamps in two situations — paragraph 167: in conditions of insufficient visibility, alone or together with dipped beam; and in the dark on unlit stretches of road, together with dipped beam only. With main beam after dark you may not: that pairing is not on the list.

In insufficient visibility, on the other hand, the rules treat them as a full main-light option: you must show dipped beam, main beam or front fog lamps, plus position lamps and the number-plate light (paragraph 162). And if in such weather both dipped-beam headlamps and both front fog lamps are out at the same time, you may not drive at all — paragraph 222.2. They help because of geometry: a low, wide beam lights not the fog ahead but the edge of the road and the markings you hold your line by in fog.

The rear lamp decides for the driver behind

With the rear fog lamp the rules are stricter, and you can see it in the wording: the front ones are allowed in insufficient visibility generally, the rear one only if there is thick fog, heavy rain or snowfall (paragraph 168). A noticeably narrower trigger.

The reason is structural. The international standard these lamps are approved against describes a rear fog lamp as a red signal of greater intensity than the rear position lamp: it is built that way to punch through a dense grey mass. In light output it is closer to a brake light than to a rear position lamp.

Hence the harm in light haze. The driver behind sees a bright red patch and builds a picture from it — "braking", or a distance. When you actually press the brake, almost nothing changes in their field of view: the brake light comes on beside a lamp of much the same brightness. Your warning signal stops being a signal precisely when it is needed.

What the rules do not say

Two things are missing from the text, and it is better to know in advance.

• No metre figure for "thick fog". Insufficient visibility is measured by the Law at three hundred metres; thick fog is not measured anywhere. The fifty metres often quoted is a European rule of thumb and insurers' advice, not a Latvian norm.
• No requirement to switch the lamp off once it clears. The construction is different: the rear lamp is allowed only while thick fog lasts. When the condition ends the permission ends with it — but the switch will not move by itself.

The judgement is therefore yours, and instead of a number a simple test works: if you can make out the tail lights of the car ahead, your fog lamp is now a nuisance to the driver behind. And check in advance that no bulb is dead: part of the ten-minute check before the dark season.

Speed comes from visibility distance, not from the sign

The rules require a speed that stays within the limits and takes road and weather conditions into account, visibility in the direction of travel above all (paragraph 99); if a hazard the driver is able to notice appears ahead, speed must be reduced, up to a full stop (paragraph 100). In practice that is a single action: if you can see fifty metres, drive so that you can stop within those fifty. The number on the sign is a ceiling and promises nothing about fog.

The other side is written down too: obstructing traffic by driving unnecessarily slowly is forbidden (paragraph 104.3). Twenty kilometres an hour on a highway in patchy fog is not caution — it is a fresh hazard.

Judging it by eye fails systematically: the brain reads speed and distance from contrast, and fog is precisely what removes contrast. The errors all run one way:

• your own speed feels lower — in fog drivers believe they are travelling markedly more slowly than they are, and speed up accordingly (psychologists at Cardiff University, Nature, 1998);
• objects look farther away than they are;
• an oncoming car looks slower than it is going.

Which is why overtaking in fog is the manoeuvre people misjudge most: the three errors add up and hand you confidence where you should have none.

Fog is rarely continuous

It gathers in patches — over hollows, water and open fields — with clear windows in between. That is the trap: in a clear window your speed climbs back on its own, and you enter the next wall faster than you entered the last. The rule is simple: a clear stretch is not the end of the fog; keep the speed of the worst stretch until you have driven several kilometres of consistently clear road.

Fog also settles on the road and on the glass: the asphalt goes damp with no rain at all and grip is worse than it looks (wet surfaces have their own post), and the wipers have to go on periodically, without waiting for rain.

What a learner loses first

In a foggy lesson the first thing to go is not car control but the sense of distance: the student keeps the same gap as in clear weather, because the car ahead is "right there" — and it is not. The second is where they look: into the grey ahead, when what is useful sits down and to the right — the edge of the lane and the markings.

Both habits can be set in a single lesson — but only if the fog is real rather than described in words.

Fog does not ask for special skill — it asks you to accept that your eyes cannot be trusted right now, and to replace judgement with arithmetic.

The wording of the rules and the Law may be amended — check the current texts on likumi.lv (verified in September 2026).