Theory Mastery

Fog: why your normal driving habits no longer apply

What the exam asks here: Most students reason: 'the ordinary 2-second gap is always good enough' and 'if the lane looks clear, I may overtake'.

14 minutes · 7 sections · 10 questions

The trap

Many learners think: In dense fog the normal two-second following distance is still sufficient.

The rule is: In very dense fog (visibility under 50 metres) three seconds, not two, applies

Challenge

Most students reason: 'the ordinary 2-second gap is always good enough' and 'if the lane looks clear, I may overtake'. That is exactly what the CBR exam designer counts on. In very dense fog the book changes both habits at once.

Read this section on its own page: Challenge: Fog: why your normal driving habits no longer apply

The model

In very dense fog (visibility under 50 metres) you adjust your driving behaviour immediately (Book p.190): keep not two but three seconds of following distance to the car ahead, reduce your speed, and preferably do not overtake — not even on a motorway (Book p.191).

Read this section on its own page: The model: Fog: why your normal driving habits no longer apply

Core

Fog forms mainly when warm, humid air flows over a cold ground surface (Book p.190). As soon as fog forms, you adjust your driving behaviour immediately. In very dense fog (visibility under 50 metres) four measures apply at once: keep three seconds' distance instead of two to the car ahead, reduce your speed, preferably do not overtake — not even on a motorway — and follow the right-hand lane as much as possible, so that in an emergency you can use the hard shoulder as a direct escape route (Book p.191). This speed reduction is independent of your lights: fog lights help others see you, but change nothing about your own, reduced sight — you reduce your speed regardless. The exact thresholds for front and rear fog lights (visibility <200 m and <50 m respectively) belong to a different atom (regen_wind.mistlicht_regel, Book p.187-188/191); this atom is about your driving behaviour once visibility is severely limited, not the precise lighting rules.

Read this section on its own page: Core: Fog: why your normal driving habits no longer apply

Why it matters

On a country road in the Drenthe, far from street lighting, visibility can drop away within a few metres — the extra second of distance is then the difference between stopping in time and a rear-end collision. Overtaking in dense fog also temporarily reduces your own view of what is happening straight ahead, exactly when you need it most.

Read this section on its own page: Why it matters: Fog: why your normal driving habits no longer apply

Active recall

On the N18 between Norg and Groenlo you drive in very dense fog, visibility under 50 metres. How many seconds of distance do you keep to the car ahead?

Read this section on its own page: Active recall: Fog: why your normal driving habits no longer apply

Teach back

Explain: which four adjustments do you make in very dense fog (visibility <50 m), and why does turning on your fog lights alone not change any of that?

Read this section on its own page: Teach back: Fog: why your normal driving habits no longer apply

Memory hook

3 seconds, not 2. Do not overtake. Right-hand lane. Fog lights are for others — you reduce your own speed regardless.

Read this section on its own page: Memory hook: Fog: why your normal driving habits no longer apply

Source: RVV 1990 art. 19 · RVV 1990 art. 32 · checked 2026-10-22

The picture

In very dense fog you change three habits at onceNormal habitIn very dense fog (<50 m, p.191)Following distance2 seconds3 secondsOvertakingallowed if the lane looks clearpreferably not, not even on the motorwayLaneusually does not matterright-hand lane, for direct hard-shoulder access
What to look at: Normal: 2 seconds' distance, overtaking allowed. In very dense fog: 3 seconds' distance, do not overtake, keep to the right-hand lane for the hard shoulder.

Signs and law

Test yourself

Early in the morning you drive through the Drenthe region near Norg. Dense fog rolls in, visibility is under 50 metres. How many seconds of following distance do you keep to the car ahead, and may you still overtake if the left-hand lane looks clear?

Show the answer

In very dense fog (visibility under 50 metres) you adjust your driving behaviour immediately (Book p.190): keep not two but three seconds of following distance to the car ahead, reduce your speed, and preferably do not overtake — not even on a motorway (Book p.191).

Exam questions on this topic

About Fog: why your normal driving habits no longer apply: which statement is WRONG?

  1. In dense fog the normal two-second following distance is still sufficient.
  2. Fog lights help others see you, but change nothing about your own reduced sight; you adjust your driving (speed, distance) regardless, once visibility is severely limited.
  3. You follow the right-hand lane as much as possible, so that in an emergency you can use the hard shoulder as a direct escape route.
  4. Referably do not overtake in dense fog, not even on a motorway.
Show the answer

Correct answer: In dense fog the normal two-second following distance is still sufficient.

In very dense fog (visibility under 50 metres) three seconds, not two, applies

Why learners get this wrong: As long as your fog lights are on, you don't need to reduce your speed further — others will see you anyway.

Open this question

In very dense fog (visibility under 50 metres) three seconds, not two, applies.

  1. False
  2. True
Show the answer

Correct answer: True

In very dense fog (visibility under 50 metres) three seconds, not two, applies

Why learners get this wrong: In dense fog the normal two-second following distance is still sufficient.

Open this question

About Fog: why your normal driving habits no longer apply: which statement is WRONG?

  1. In very dense fog (visibility under 50 metres) three seconds, not two, applies.
  2. As long as your fog lights are on, you don't need to reduce your speed further — others will see you anyway.
  3. Referably do not overtake in dense fog, not even on a motorway.
  4. You follow the right-hand lane as much as possible, so that in an emergency you can use the hard shoulder as a direct escape route.
Show the answer

Correct answer: As long as your fog lights are on, you don't need to reduce your speed further — others will see you anyway.

Fog lights help others see you, but change nothing about your own reduced sight; you adjust your driving (speed, distance) regardless, once visibility is severely limited

Why learners get this wrong: In dense fog the normal two-second following distance is still sufficient.

Open this question

Fog lights help others see you, but change nothing about your own reduced sight; you adjust your driving (speed, distance) regardless, once visibility is severely limited.

  1. False
  2. True
Show the answer

Correct answer: True

Fog lights help others see you, but change nothing about your own reduced sight; you adjust your driving (speed, distance) regardless, once visibility is severely limited

Why learners get this wrong: As long as your fog lights are on, you don't need to reduce your speed further — others will see you anyway.

Open this question

On the N18 near Groenlo you drive in very dense fog, visibility under 50 metres. How many seconds of distance do you keep to the car ahead?

Show the answer

Correct answer: 3 sec

Correct: 3.0 sec (±0.5).

Open this question

On the A7 near Rolde you drive in dense fog. The left-hand lane looks completely clear. Statement: you may simply overtake here.

  1. True
  2. False
Show the answer

Correct answer: False

Why learners get this wrong: On the motorway you may simply overtake in dense fog if the left-hand lane looks clear.

Open this question

On the A28 between Meppel and Zwolle there is very dense fog. You are already driving slower than normal on a road with two lanes and a hard shoulder. Which lane do you choose?

  1. It does not matter which, as long as I drive clearly slower than normal
  2. The left one, so that I can overtake slow vehicles ahead smoothly in the fog
  3. The right one, so that in an emergency I can swerve straight onto the hard shoulder
Show the answer

Correct answer: The right one, so that in an emergency I can swerve straight onto the hard shoulder

False — You apply the ordinary overtaking habit in fog. Preferably do not overtake in dense fog, not even on a motorway, and keep to the right as much as possible

Why learners get this wrong: In fog it does not matter which lane you follow, as long as you drive more slowly.

Open this question

Which adjustments does the book say you should make in very dense fog (visibility <50 m)? (multiple answers possible)

  1. Not overtake, not even on the motorway
  2. Reduce speed
  3. Keep to the right-hand lane
  4. Keep three seconds' distance instead of two
  5. Simply keep your normal speed as long as your fog lights are on
Show the answer

Correct answer: Not overtake, not even on the motorway; Reduce speed; Keep to the right-hand lane; Keep three seconds' distance instead of two

Why learners get this wrong: As long as your fog lights are on, you don't need to reduce your speed further — others will see you anyway.

Open this question

On an autumn evening near Assen, warm, humid air drifts over the cold meadows beside the road. Within a minute you are driving in thick fog and can see less than 50 metres. There is a car in front of you. What do you do?

  1. Fog lights on, drive more slowly and keep a three-second gap
  2. Fog lights on, drive more slowly and keep a two-second gap
  3. Fog lights on; then others see you and you simply keep your pace
Show the answer

Correct answer: Fog lights on, drive more slowly and keep a three-second gap

False — the usual two seconds are too little in fog like this. With visibility under 50 metres you keep at least three seconds from the vehicle ahead and reduce your speed

Why learners get this wrong: In dense fog the normal two-second following distance is still sufficient.

Open this question

Near Heerenveen you drive through fog in the morning. You can see about 80 metres: limited, but not yet below 50 metres. What do you do?

  1. Switch on my front fog light and drive on, because then others see me
  2. Drive on with a 2-second gap; adjusting is only needed below 50 metres
  3. Lower my speed now and keep more distance than in good visibility
Show the answer

Correct answer: Lower my speed now and keep more distance than in good visibility

False — You wait to adjust until the 50-metre limit. When fog forms you adjust your driving straight away; below 50 metres of visibility you even keep three instead of two seconds of distance

Why learners get this wrong: As long as your fog lights are on, you don't need to reduce your speed further — others will see you anyway.

Open this question

Frequently asked questions

‘In dense fog the normal two-second following distance is still sufficient’ – is that true?

No. In very dense fog (visibility under 50 metres) three seconds, not two, applies

‘On the motorway you may simply overtake in dense fog if the left-hand lane looks clear’ – is that true?

No. Referably do not overtake in dense fog, not even on a motorway

‘In fog it does not matter which lane you follow, as long as you drive more slowly’ – is that true?

No. You follow the right-hand lane as much as possible, so that in an emergency you can use the hard shoulder as a direct escape route

‘As long as your fog lights are on, you don't need to reduce your speed further — others will see you anyway’ – is that true?

No. Fog lights help others see you, but change nothing about your own reduced sight; you adjust your driving (speed, distance) regardless, once visibility is severely limited

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