Theory Mastery

The braking-distance rule of thumb: (speed:10)² : 2

What the exam asks here: Most learners calculate: '80 divided by 10 is 8, squared is 64...

14 minutes · 7 sections · 8 questions

The trap

Many learners think: Braking distance = (speed:10) squared, with no further division (so 25 m at 50 km/h, 100 m at 100 km/h).

The rule is: The rule of thumb literally is '(speed(km/h):10) x (speed(km/h):10) : 2', with the book's own worked examples 50 km/h = 12.50 m, 80 km/h = 32.00 m, 130 km/h = 84.50 m. Without the :2, every figure is exactly double the book value

Challenge

Most learners calculate: '80 divided by 10 is 8, squared is 64... no wait, 32 metres?' Half of them forget one step in the formula — and that step is exactly what the CBR exam designer counts on. The book does not give a bare square. It gives a square, divided by 2.

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The model

Braking distance = (speed:10) x (speed:10) : 2 (Book p.126). At 80 km/h: (8 x 8) : 2 = 32.00 m. Not 64 m — the division by 2 belongs to the formula itself, not a separate correction.

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Core

The formula on p.126 has two steps: first divide speed by 10 and square it, then divide that result by 2. At 130 km/h: (13 x 13) : 2 = 84.50 m. At 50 km/h: (5 x 5) : 2 = 12.50 m. Braking distance is not the same as stopping distance. Stopping distance = reaction distance + braking distance. At 50 km/h that is 13.75 m (reaction distance) + 12.50 m (braking distance) = 26.25 m total — almost double the braking distance alone, because the reaction distance is added on top. The ratio stays quadratic, even without the :2 mistake: drive 2x as fast, and your braking distance becomes 4x as long. At 40 km/h braking distance is 8.00 m; at 80 km/h (2x as fast) it is 32.00 m — exactly 4x.

Read this section on its own page: Core: The braking-distance rule of thumb: (speed:10)² : 2

Why it matters

Forgetting the :2 makes you overestimate your own braking distance — that sounds safe, but in practice leads to overly cautious, unpredictable braking behaviour and wrong estimates on the exam. Dropping it the other way when judging following distance makes you underestimate it and tailgate. Both sides of the mistake cost points on the CBR exam, which tests the exact book values.

Read this section on its own page: Why it matters: The braking-distance rule of thumb: (speed:10)² : 2

Active recall

On the A35 near Nijverdal you are driving 100 km/h in the right-hand lane. What is your braking distance per the book formula?

Read this section on its own page: Active recall: The braking-distance rule of thumb: (speed:10)² : 2

Teach back

Explain: what are the two steps of the braking-distance rule of thumb, and why is 25 metres at 50 km/h the wrong answer?

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Memory hook

(speed:10)² : 2. 50→12.50m · 80→32.00m · 130→84.50m. Don't forget the :2 — otherwise your braking distance looks double on paper.

Read this section on its own page: Memory hook: The braking-distance rule of thumb: (speed:10)² : 2

The picture

Braking-distance rule of thumb: (speed:10)² : 2 — the :2 is not an extra correction50 km/h80 km/hReaction distance (p.125)13.75 m22.00 mBraking distance (p.126)12.50 m32.00 mStopping distance (p.126)26.25 m54.00 m
What to look at: 50 km/h: 13.75 / 12.50 / 26.25 m. 80 km/h: 22.00 / 32.00 / 54.00 m. 130 km/h: 35.75 / 84.50 / 120.25 m.

Test yourself

On the A35 near Nijverdal you are driving 80 km/h. Two cars ahead, the front one suddenly brakes for a traffic jam. How many metres of braking distance does your car need to come to a stop, per the book's rule of thumb?

Show the answer

Braking distance = (speed:10) x (speed:10) : 2 (Book p.126). At 80 km/h: (8 x 8) : 2 = 32.00 m. Not 64 m — the division by 2 belongs to the formula itself, not a separate correction.

Exam questions on this topic

About The braking-distance rule of thumb: (speed:10)² : 2: which statement is WRONG?

  1. If speed doubles, braking distance also doubles (a linear relationship).
  2. The rule of thumb literally is '(speed(km/h):10) x (speed(km/h):10) : 2', with the book's own worked examples 50 km/h = 12.50 m, 80 km/h = 32.00 m, 130 km/h = 84.50 m.
  3. Stopping distance = reaction distance + braking distance.
Show the answer

Correct answer: If speed doubles, braking distance also doubles (a linear relationship).

'braking distance increases progressively: if you drive 2x as fast, your braking distance becomes 4x as long.' At 40 km/h braking distance is 8.00 m; at 80 km/h (2x as fast) it is 32.00 m — 4x, not double

Why learners get this wrong: Braking distance = (speed:10) squared, with no further division (so 25 m at 50 km/h, 100 m at 100 km/h).

Open this question

On the A35 near Nijverdal you are driving 100 km/h in the right-hand lane. What is your braking distance per the book formula?

  1. 50.00 m
  2. 100.00 m
  3. 200.00 m
Show the answer

Correct answer: 50.00 m

(speed:10)² : 2 — don't forget the division: (10x10):2 = 50.00 m

Why learners get this wrong: Braking distance = (speed:10) squared, with no further division (so 25 m at 50 km/h, 100 m at 100 km/h).

Open this question

On the A35 near Nijverdal you are driving 60 km/h. Per the book formula (speed:10)² : 2, what is your braking distance in metres?

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Correct answer: 18 m

Correct: 18.0 m (±2.0).

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On a provincial road near Wierden you are driving 100 km/h. What is your braking distance per the book formula?

  1. 50.00 m
  2. 100.00 m
  3. 200.00 m
Show the answer

Correct answer: 50.00 m

Why learners get this wrong: Braking distance = (speed:10) squared, with no further division (so 25 m at 50 km/h, 100 m at 100 km/h).

Open this question

In the built-up area of Hellendoorn you explain to a passenger: 'If you accelerate from 50 km/h to 100 km/h, your braking distance becomes exactly 2x as long.'

  1. True
  2. False
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Correct answer: False

Why learners get this wrong: If speed doubles, braking distance also doubles (a linear relationship).

Open this question

You are driving at 80 km/h on the road between Nijverdal and Wierden. You see that the car ahead has to brake hard. How long is your stopping distance, from the moment you see it until you stand still?

  1. About 54 metres
  2. About 32 metres
  3. About 86 metres
Show the answer

Correct answer: About 54 metres

False — you calculate the braking distance without dividing by 2. Braking distance = (speed : 10) x (speed : 10) : 2, so at 80 km/h (8 x 8) : 2 = 32 m, not 64 m. Adding about 22 m of reaction distance, the stopping distance is 54 m

Why learners get this wrong: The stopping distance at 50 km/h is the same as the braking distance: 12.50 m.

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Near Goor you are driving 130 km/h in the evening with no traffic ahead in your lane. What is your braking distance per the book formula?

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Correct answer: 84.5 m

Correct: 84.5 m (±3.0).

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On the A35 you read on an instruction card: 'At 50 km/h the stopping distance is exactly equal to the braking distance: 12.50 m.'

  1. True
  2. False
Show the answer

Correct answer: False

Why learners get this wrong: The stopping distance at 50 km/h is the same as the braking distance: 12.50 m.

Open this question

Frequently asked questions

‘Braking distance = (speed:10) squared, with no further division (so 25 m at 50 km/h, 100 m at 100 km/h)’ – is that true?

No. The rule of thumb literally is '(speed(km/h):10) x (speed(km/h):10) : 2', with the book's own worked examples 50 km/h = 12.50 m, 80 km/h = 32.00 m, 130 km/h = 84.50 m. Without the :2, every figure is exactly double the book value

‘If speed doubles, braking distance also doubles (a linear relationship)’ – is that true?

No. 'braking distance increases progressively: if you drive 2x as fast, your braking distance becomes 4x as long.' At 40 km/h braking distance is 8.00 m; at 80 km/h (2x as fast) it is 32.00 m — 4x, not double

‘The stopping distance at 50 km/h is the same as the braking distance: 12.50 m’ – is that true?

No. Stopping distance = reaction distance + braking distance. At 50 km/h: 13.75 m (reaction distance) + 12.50 m (braking distance) = 26.25 m stopping distance, more than double the braking distance alone

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