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Reaction: you haven’t touched the brake yet Braking
An estimate for teaching. It uses standard physics with the Federal Highway Administration’s braking values (emergency stop on dry pavement 0.7g, wet 0.4g, snow 0.22g, ice 0.15g) and starts at the 2.5-second design reaction time. Real stopping distances vary with the vehicle, tires, brakes, load, slope and driver.
Try these
- School zone vs. main road. At 30 mph on dry pavement, stopping takes about 153 feet. At 60 mph it takes about 392 feet: twice the speed, well over twice the distance.
- Same street, icy morning. At 30 mph on ice, the estimate jumps to about 311 feet.
- Add one second of distraction. Move reaction time from 2.5 to 3.5 seconds and watch the red part of the bar grow.
How it works
Stopping takes two parts. Reaction distance is how far you travel while you notice the hazard and move your foot: speed × reaction time. Braking distance is how far the car slides or rolls after the brakes are on: speed² ÷ (2 × friction × gravity). Because speed is squared, braking distance grows much faster than speed.
Fun facts (and not-so-fun facts)
- Speeding was a factor in 29% of all US traffic deaths: 11,288 people.NHTSA, 2024
- For more than 20 years, speeding has been involved in about one-third of all motor vehicle deaths.NHTSA, two-decade trend
- Highway engineers design stopping distances assuming a 2.5-second perception-reaction time, a cautious value that covers most drivers.FHWA, AASHTO design value
Use it in a class or workshop
Have each group guess the distance at 25, 45 and 65 mph, then check with the simulator. Pace out the 25 mph answer in a parking lot or hallway. The full lesson is in the teen and new driver workshop kit.
Sources: Federal Highway Administration, Speed Concepts Informational Guide, Chapter 4 (AASHTO 2.5-second perception-reaction time; deceleration values), checked November 7, 2026.
Questions people ask us
How long does it take to stop a car at 60 mph?
With the 2.5-second reaction time highway engineers design for, on dry pavement, about 392 feet: 220 feet before your foot reaches the brake and 172 feet of braking. That is an estimate; real distances depend on the car, tires, brakes, slope and the driver.
Why does doubling speed more than double stopping distance?
Braking distance grows with the square of speed. Twice the speed needs about four times the braking distance, while the reaction distance only doubles.
What is reaction time?
The time between seeing a hazard and pressing the brake. Highway engineers design roads assuming 2.5 seconds, a cautious value that covers most drivers. Alert drivers who expect a stop can be faster; distraction, fatigue, alcohol and drugs make it slower.
