# Stopping distance = reaction distance + braking distance ## Reaction distance

The reaction distance is the distance you travel from the point of detecting a hazard until you begin braking or swerving.

### The reaction distance is affected by

• The car’s speed (proportional increase):
• 2 x higher speed = 2 x longer reaction distance.
• 5 x higher speed = 5 x longer reaction distance.
• Normally 0.5–2 seconds.
• 45–54 year-olds have the best reaction time in traffic.
• 18–24 year-olds and those over 60 have the same reaction time in traffic. Young people have sharper senses but older people have more experience.

### The reaction distance can be decreased by

• Anticipation of hazards.
• Preparedness.

### Easy method: Calculate the reaction distance

Formula: Remove the last digit in the speed, multiply by the reaction time and then by 3.

Example of calculation with a speed of 50 km/h and a reaction time of 1 second:

50 km/h ⇒ 5
5 * 1 * 3 = 15 metres reaction distance

### More precise method: Calculate the reaction distance

Formula: d = (s * r) / 3.6

d = reaction distance in metres (to be calculated).
s = speed in km/h.
r = reaction time in seconds.
3.6 = fixed figure for converting km/h to m/s.

Example of calculation with a speed of 50 km/h and a reaction time of 1 second:

(50 * 1) / 3.6 = 13.9 metres reaction distance

## Braking distance

The braking distance is the distance the car travels from the point when you start braking until the car stands still.

### The braking distance is affected by

• The vehicle’s speed (quadratic increase; “raised to the power of 2”):
• 2 x higher speed = 4 x longer braking distance.
• 3 x higher speed = 9 x longer braking distance.
• The brakes (condition, braking technology and how many wheels are braking).

### Calculate the braking distance

It is very difficult to achieve reliable calculations of the braking distance as road conditions and the tyres’ grip can vary greatly. The braking distance may for example be 10 times longer when there is ice on the road.

### Easy method: Calculate the braking distance

Conditions: Good and dry road conditions, good tyres and good brakes.

Formula: Remove the zero from the speed, multiply the figure by itself and then multiply by 0.4.

The figure 0.4 is taken from the fact that the braking distance from 10 km/h in dry road conditions is approximately 0.4 metres. This has been calculated by means of researchers measuring the braking distance. Thus, in the simplified formula, we base our calculations on the braking distance at 10 km/h and increase it quadratically with the increase in speed.

Example of calculation with a speed of 10 km/h:

10 km/h ⇒ 1
1 * 1 = 1
1 * 0.4 = 0.4 metres braking distance

Example of calculation with a speed of 50 km/h:

50 km/h ⇒ 5
5 * 5 = 25
25 * 0.4 = 10 metres braking distance

### More precise method: Calculate the braking distance

Conditions: Good tyres and good brakes.

Formula: d = s2 / (250 * f)

d = braking distance in metres (to be calculated).
s = speed in km/h.
250 = fixed figure which is always used.
f = coefficient of friction, approx. 0.8 on dry asphalt and 0.1 on ice.

Example of calculation with a speed of 50 km/h on dry asphalt:

502 / (250 * 0.8) = 12.5 metres braking distance

## Stopping distance

Stopping distance = reaction distance + braking distance

### Calculate the stopping distance with these easy methods

It is summer and the road is dry. You are driving at 90 km/h with a car with good tyres and brakes. You suddenly notice a hazard on the road and brake forcefully. How long is the stopping distance if your reaction time is 1 second?

The stopping distance is the reaction distance + braking distance. First we calculate the reaction distance:

• 90 km/h ⇒ 9
• 9 * 1 * 3 = 27 metres reaction distance

Then we calculate the braking distance:

• 90 km/h ⇒ 9
• 9 * 9 = 81
• 81 * 0.4 = 32 metres braking distance

Now both distances are combined:

• 27 + 32 = metres stopping distance

### Important clarification regarding calculations

The different methods provide different answers. Which should I use?
– Use whichever you wish. The differences are so small that they will not affect your theory test, as the margins between the alternatives are quite large.

So if the alternatives are 10, 20, 40, 60, it does not matter if you get 10 metres with one method and 12.5 metres with another – both are obviously closest to 10, which is thus the right answer.

Last updated 2021-01-22.