5.2 Stopping Distances & Speed
Key Takeaways
- Total stopping distance = thinking distance + braking distance
- Thinking distance rises in direct proportion to speed; braking distance rises with the square of speed
- Doubling your speed roughly quadruples your braking distance
- Wet roads roughly double stopping distances; worn tyres, worn brakes, heavy loads, fatigue, alcohol, and phone use all add metres
- Ghana's 50 km/h urban limit — and 30 km/h near schools — exists because small speed differences mean large differences in whether a pedestrian survives
The Two Parts of Stopping Distance
When you see a hazard and stop, your vehicle travels two distinct distances:
- Thinking distance — the distance your vehicle travels between the moment you see the hazard and the moment your foot presses the brake. At typical alert reaction times (around 0.7–1 second), the car keeps moving at full speed the whole time.
- Braking distance — the distance travelled from brake application to a full stop.
Total stopping distance = thinking distance + braking distance. Exam questions love this definition, so memorise it exactly.
How Speed Changes Each Part
The two parts behave very differently as speed rises:
- Thinking distance rises in direct (linear) proportion to speed. Double your speed and you travel twice as far while reacting. At 30 km/h a typical thinking distance is roughly 6 metres; at 60 km/h it is about 12 metres.
- Braking distance rises with the SQUARE of speed. Double your speed and braking distance becomes about four times longer. If you need roughly 6 metres to brake from 30 km/h, you will need around 24 metres from 60 km/h — not 12.
Put together, doubling from 30 km/h to 60 km/h grows total stopping distance from roughly 12 metres to roughly 36 metres — a threefold increase from a mere doubling of speed. This squared relationship is the single most important physics fact in defensive driving, and it is why "I was only going a little over the limit" is such a dangerous excuse near pedestrians.
Rule of thumb for the exam: speed × 2 → braking distance × 4.
A Worked Example
Picture two identical drivers on the same dry urban road who both see a child step into the road 25 metres ahead. The first is at 30 km/h: about 6 metres of thinking distance plus about 6 metres of braking distance means the car stops with roughly 13 metres to spare. The second driver is at 60 km/h: around 12 metres of thinking plus around 24 metres of braking is 36 metres in total — the car is still moving at speed when it reaches the child's position. Same driver, same car, same road; the only difference is the speed, and the squared braking effect turned a routine stop into a tragedy. This is precisely why exam questions frame speed choices in terms of stopping distance, and why the defensive driver treats every speed decision as a stopping-distance decision.
Factors That Increase Stopping Distance
| Factor | Which part it affects | Effect |
|---|---|---|
| Wet or slippery road (rain, mud, oil) | Braking | Roughly doubles braking distance |
| Loose dust or gravel on laterite roads | Braking | Longer, less predictable braking |
| Worn or under-inflated tyres | Braking | Reduced grip, longer stops |
| Worn brakes | Braking | Weaker, longer stops; possible fade |
| Heavy load / many passengers | Braking | More momentum to stop |
| Fatigue | Thinking | Slower reactions, longer thinking distance |
| Alcohol (legal limit is 0.05% BAC) | Thinking | Slower, poorer reactions even below the limit |
| Mobile phone / other distractions | Thinking | Reaction delayed or hazard never seen |
Notice the pattern: driver-condition factors (fatigue, alcohol, phone) stretch the thinking distance, while vehicle and road factors (tyres, brakes, wet surface, load) stretch the braking distance. Speed stretches both.
Why Ghana's Urban Limits Are Set Where They Are
Ghana's general speed limit in built-up areas is 50 km/h, dropping to 30 km/h in school and hospital zones. The stopping-distance maths explains both numbers:
- At 50 km/h, an alert driver on a dry road still needs well over 20 metres to stop — and urban roads are exactly where pedestrians, okada riders, and tro-tro stops create sudden hazards.
- At 30 km/h, the total stopping distance shrinks dramatically (in the region of 12 metres in good conditions), and — just as important — a struck pedestrian's survival chances are far higher at 30 km/h than at 50 km/h.
Remember also that these are limits, not targets. In rain, in crowds, near a school at closing time, the safe speed may be far below the posted limit. The defensive driver always chooses a speed that allows stopping within the clear distance visible ahead.
Practical Applications
- Leave margin for the squared effect. Small increases in speed buy almost no time on a journey but cost large increases in stopping distance.
- Double your following gap on wet roads, because braking distances roughly double — this is the four-second rule from Section 5.1.
- Maintain your vehicle. Tyres with legal tread depth and healthy brakes are stopping-distance equipment, not comforts.
- Protect your thinking distance. Never drive tired, never drink and drive, and put the phone away — a delayed reaction at 50 km/h adds metres before the brakes even engage.
A driver brakes from 40 km/h on a dry road and needs about 9 metres of braking distance. Travelling at 80 km/h on the same road, approximately what braking distance should the driver expect?
Which of the following mainly increases a driver's thinking distance rather than the braking distance?