Stopping distance formula, explained step by step
Home › Stopping distance formula, explained step by step
Stopping distance comes up in every Belgian theory test, either as a calculation or as a situation: “at 90 km/h on a wet road, can you stop before the obstacle?” To answer without hesitation, you only need two ideas: what happens while you react, and why braking distance grows much faster than speed.
Key points
Stopping = thinking + braking
Total stopping distance is the distance covered during your reaction time plus the braking distance.
Braking grows with the square
Twice the speed means four times the braking distance. This is the most frequently tested idea.
On wet roads, double it
On a wet surface, allow roughly twice the braking distance you would need on a dry road.
Three distances you must not confuse
Reaction (thinking) distance is the distance travelled between seeing the hazard and your foot actually pressing the brake pedal. During that time the car is still moving at full speed. For an alert driver it takes about one second; fatigue, alcohol, a phone or a moment of distraction make it longer.
Braking distance is the distance from the moment braking starts until the car comes to a complete stop. It depends on speed, road surface, tyres and brakes. Stopping distance is the sum of the two, and it decides whether you can avoid an obstacle.
| Distance | Depends on | Made longer by |
|---|---|---|
| Reaction | Speed and reaction time | Fatigue, alcohol, phone, medication |
| Braking | Speed squared, grip | Rain, ice, worn tyres, heavy load |
| Stopping | Reaction + braking | All of the above |
The stopping distance formula
With speed v in metres per second (km/h divided by 3.6), reaction distance is v × reaction time and braking distance is v² ÷ (2 × deceleration). On a dry road, a car in good condition decelerates at about 7 m/s².
Example at 90 km/h: 90 ÷ 3.6 = 25 m/s. Reaction: 25 × 1 = 25 m. Braking: 25² ÷ 14 ≈ 45 m. Stopping distance: about 70 metres, more than half a football pitch.
| Speed | Reaction | Braking | Stopping |
|---|---|---|---|
| 30 km/h | ≈ 8 m | ≈ 5 m | ≈ 13 m |
| 50 km/h | ≈ 14 m | ≈ 14 m | ≈ 28 m |
| 70 km/h | ≈ 19 m | ≈ 27 m | ≈ 46 m |
| 90 km/h | ≈ 25 m | ≈ 45 m | ≈ 70 m |
| 120 km/h | ≈ 33 m | ≈ 80 m | ≈ 113 m |
The mental shortcut taught by Belgian driving schools
To calculate in your head, driving schools use a simple approximation. Take the tens digit of your speed (90 km/h → 9).
- Reaction distance ≈ tens × 3 (at 90 km/h: 9 × 3 = 27 m).
- Braking distance on a dry road ≈ tens squared ÷ 2 (at 90 km/h: 81 ÷ 2 ≈ 40 m).
- On a wet road: double the braking distance (at 90 km/h: ≈ 80 m).
- Stopping distance = reaction + braking (at 90 km/h, dry: ≈ 67 m).
These are orders of magnitude. In the exam, what matters is the reasoning: reaction distance grows with speed, braking distance with its square.
What the theory test really asks
Questions rarely ask for an exact number. They test relationships: what happens to braking distance when speed doubles? (it quadruples). What makes reaction distance longer? (fatigue, alcohol, distraction, not the road surface). What makes braking distance longer? (speed, rain, worn tyres, not reaction time).
The practical consequence is following distance: keep at least two seconds behind the vehicle in front, four in the rain. See also the speed limits in Belgium.
Practise this topic with real exam questions
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Frequently asked questions
What is the stopping distance formula?
Stopping distance = reaction distance + braking distance. With v in m/s: reaction = v × reaction time; braking = v² ÷ (2 × deceleration).
What is the stopping distance at 50 km/h?
About 28 metres on a dry road: 14 m reaction and 14 m braking. On a wet road, allow around 40 metres.
Does braking distance double when speed doubles?
No, it quadruples, because it depends on the square of the speed. Reaction distance does double.
What increases reaction distance?
Anything that slows the driver down: fatigue, alcohol, drugs, some medicines, a phone, distraction. The road surface only affects braking distance.
What is the two-second rule?
Pick a fixed point by the road. When the vehicle ahead passes it, count “one thousand and one, one thousand and two”. If you reach the point before you finish, you are too close.