Two parts of every stop
The distance a car needs to stop is the sum of:
- Reaction distance: the ground covered between seeing the hazard and pressing the brake. It grows in proportion to speed.
- Braking distance: the ground covered while the brakes slow the car. It grows with the square of speed.
The formulas
- Reaction distance = v × t, where t is the reaction time.
- Braking distance = v² ÷ (2 × g × (μ − grade)), where μ is the tire-road friction coefficient, g = 9.81 m/s² and the grade is positive downhill.
Worked example at 60 mph
With a reaction time of 1.5 seconds on dry asphalt (μ = 0.7):
- 60 mph = 26.8 m/s (88 ft/s).
- Reaction distance = 88 × 1.5 = 132 ft.
- Braking distance = 26.8² ÷ (2 × 9.81 × 0.7) ≈ 52.4 m = 172 ft.
- Total ≈ 304 ft (93 m), about the length of a football field.
On a wet road (μ = 0.4), braking alone grows to about 300 ft, and the total exceeds 430 ft.
Stopping distances on dry pavement
| Speed | Reaction (1.5 s) | Braking | Total |
|---|---|---|---|
| 25 mph | 55 ft | 30 ft | 85 ft |
| 35 mph | 77 ft | 59 ft | 136 ft |
| 45 mph | 99 ft | 97 ft | 196 ft |
| 55 mph | 121 ft | 144 ft | 265 ft |
| 65 mph | 143 ft | 202 ft | 345 ft |
| 75 mph | 165 ft | 269 ft | 434 ft |
Going from 55 to 75 mph adds 169 ft to the stop, more than the length of ten cars.
Reaction time
An alert driver who expects a hazard can react in about 0.75 seconds; 1.5 seconds is a realistic value for a surprise event, and 2 to 2.5 seconds is common with distraction, fatigue or alcohol. Looking at a phone for 2 seconds at 60 mph means 176 ft driven blind.
The 3-second rule
Most state driver manuals recommend a following distance of at least 3 seconds, and more in rain, at night, behind motorcycles and large trucks or when towing. At 60 mph, 3 seconds is about 264 ft. The gap does not cover your full stopping distance, because the car ahead also needs room to stop, but it gives you time to react.
What lengthens the distance
Worn tires, under-inflated tires, heavy loads, downhill grades, gravel, wet leaves, snow and ice all reduce effective friction. ABS helps you steer while braking but does not always shorten the distance, especially on loose snow or gravel.