Stopping Distances and the Severity of Crashes: Why 5 mph Matters

Stopping distances and the severity of crashes are linked by one fact: braking distance grows with the square of speed.So a small increase in speed doesn’t just mean stopping a little later. If something appears at the distance you would have needed at the lower speed, you reach it while still travelling surprisingly fast — and the energy of the impact rises even faster than the speed.

The 30 mph vs 35 mph example

Take an alert driver (0.67 s thinking time, as in the Highway Code figures) on a dry road. From 30 mph the car stops in about23 m (75 ft). Now start the same stop at 35 mph. The car covers more ground while the driver reacts, then needs more room to brake — and at the point where the 30 mph car came to rest, it is still doing about 21 mph. From 40 mph it would still be doing about 30 mph there.

The effect is just as stark at low speeds, which is why school zones and residential streets often have 20 mph limits. From 20 mph a car stops in about 12 m (40 ft):

Why a little more speed makes a big difference

• Thinking distance happens at full speed. During the reaction time you haven’t slowed at all, so a faster car is already further down the road when braking begins.
• Braking distance grows with speed squared. The brakes have to absorb the car’s kinetic energy, ½mv². Going from 30 to 35 mph raises that energy by about 36%; doubling your speed quadruples it.
• Most of the speed comes off at the end. Under steady braking a car loses speed slowly over the first part of the stop and quickly over the last part, so a car that runs out of room even a short way before its stopping point still carries a lot of speed.

Impact speed and the risk of injury

The energy a crash delivers is what injures people, and injury risk climbs steeply with impact speed. A widely cited AAA Foundation study of US crashes estimated the average risk to a pedestrian struck by a car:

Put the two tables together and the point is clear: the 21 mph impact from a 35 mph start sits right where pedestrian risk is rising fastest, while the car that started at 30 mph never hit anything at all.

Reaction time matters too

Speed isn’t the only lever. A distracted or tired driver who reacts a second later travels another 44 feet at 30 mph — at full speed — before braking even starts. Find out how quickly you react with the reaction time test, then plug your own number into the stopping distance calculator to see what it means at different speeds.

How to cut your stopping distance — and crash risk

• Slow down where hazards appear suddenly: near schools, parked cars, junctions and pedestrian crossings.
• Leave a bigger gap — at least three seconds in good conditions, more in the wet and behind large vehicles.
• Stay alert: put the phone away and take breaks on long drives; fatigue slows reactions as much as alcohol can.
• Cover the brake in busy streets so your foot is already in place.
• Keep tires and brakes in good condition, and double your expectations of braking distance on wet roads.

Heavier vehicles need even more room; see the semi truck stopping distance chart for how an 18-wheeler compares with a car.