A driver doubles their speed from 30 mph to 60 mph on the same dry stretch of road. What happens to the distance required to stop?

Why A is correct

Reaction distance scales linearly with speed, so doubling speed doubles the ground covered before the driver has even touched the pedal. Braking distance is worse still: the kinetic energy the brakes must shed rises with the square of velocity, so twice the speed carries four times the energy and demands roughly four times the braking distance. Combined, stopping from sixty takes several times what stopping from thirty did on the same road with the same tyres. This is why pedestrian survivability falls off so steeply across modest increases in impact speed.

Why the other options are wrong

B. It falls slightly, because speed helps the tyres bite

Nothing about higher speed helps a tyre grip. Tyre load sensitivity means available grip per unit of load tends to fall as forces rise, moving the wrong way entirely.

C. It roughly doubles

Linear scaling describes reaction distance alone. The braking component is what actually dominates the total, and it follows entirely different rules. Assuming the whole distance scales the way thinking distance does leaves a driver consistently under-braking for the space genuinely available ahead of them.

D. It stays much the same, since the brakes do the work

The brakes do perform the work, but the work scales with energy, and energy is not linear in speed. Same pads, same discs, four times the energy to dispose of.

On the road

Two drivers brake for the same obstacle, one travelling at thirty and one at sixty. The first stops with room to spare. The second needs several times the distance on identical road, and there is not that much road. Nothing about better brakes or newer tyres changes the underlying relationship; they only alter how sharply it bites.

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