Skidder

Meet Skidder, a delivery van who learned that doubling your speed does far more than double your stopping distance.

When the brakes lock or work at their limit, friction between the tyres and the road is what slows the van. The friction force is μmg, where μ is the coefficient of friction. That force must remove all of the van's kinetic energy, ½mv², and the van's mass cancels out.

The braking distance is d = v²/(2μg). On dry asphalt with μ ≈ 0.7, a van at 20 m/s, about 72 km/h, needs d = 400/(2 × 0.7 × 9.81) ≈ 29 m.

At 40 m/s the distance is four times larger, about 117 m. On a wet road where μ drops to about 0.4, even 20 m/s needs about 51 m.

Driver reaction time adds more distance before the brakes even start working. That is why speed limits drop near schools and why you should leave a bigger gap in the rain.