How the math works
Total stopping distance has two parts:
- Reaction distance = speed × reaction time. At 60 mph you travel about 88 feet per second, so a 1.5-second reaction costs 132 feet before the brakes do anything.
- Braking distance = v² / (2μg), where v is speed, μ is tire-road grip, and g is gravity. Because speed is squared, doubling your speed quadruples the braking distance.
The speed-squared trap
Going from 30 to 60 mph doubles your reaction distance but multiplies braking distance by four. That is why the difference between a fender-bender and a pedestrian fatality is often just a few mph in a neighborhood.
Conditions change grip dramatically
Dry pavement grips at roughly μ ≈ 0.7. Wet roads cut that by about a third, packed snow by more than half, and ice by nearly 90%. The same 60 mph stop that takes about 300 feet on dry pavement can take well over 1,000 feet on ice - more than three football fields.
Pair this with the reaction time test to see how your own response time shifts the numbers.