Free · Physics models
Road Stopping Distance and Constant Braking Time
Estimate a captured empirical reaction-plus-braking distance and a separate constant-deceleration braking interval.
- Formula & worked example
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Calculator inputs
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How to use this calculator
- Enter the known values in the units shown. Results update as you type.
- Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
- Use the worked example to check the method. Reset restores the starting fields.
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Formula and method
At 36 kilometers per hour on the captured dry-pavement model, a one-second reaction gives about 17.30 meters of stopping distance on level ground. The separately entered 5-meter-per-second-squared deceleration gives a two-second braking interval.
Distance = 0.278×reaction seconds×speed km/h + speed²/[254×(friction+grade)]. Corrected braking time = speed km/h/(3.6×entered deceleration m/s²).
Worked example
Enter these known values and leave the other values blank.
- Vehicle speed
- 36 km/h
- Perception-reaction time
- 1 sec
- Deceleration
- 5 m/s²
- Friction coefficient
- 0.7
- Stopping distance
- 17.297 m
- Braking time
- 2 sec
Assumptions and limitations
- The captured pavement model uses dry friction 0.7 or a speed-dependent wet-pavement fit. Positive grade is uphill and negative grade is downhill; friction plus grade must remain positive.
- The distance equation retains captured rounded metric constants and an empirical friction fit. It is not a road-design approval or a prediction for every vehicle, tire or surface.
- The braking-time result uses the separately entered constant deceleration and excludes reaction time. This corrects the captured use of total stopping distance inside a braking-time square root. Distance and time need not describe the same braking performance unless their assumptions agree.
- These formulas calculate results from entered inputs; a missing input can prevent a result.
Common questions
What does this scenario assume?
The captured pavement model uses dry friction 0.7 or a speed-dependent wet-pavement fit. Positive grade is uphill and negative grade is downhill; friction plus grade must remain positive.
What does the result leave out?
The braking-time result uses the separately entered constant deceleration and excludes reaction time. This corrects the captured use of total stopping distance inside a braking-time square root. Distance and time need not describe the same braking performance unless their assumptions agree.
References
- FHWA: reaction and braking contributions to stopping sight distance
- OpenStax: constant acceleration velocity and time
- Calculation definition and unit reference
Reviewed local arithmetic: Distance = 0.278×reaction seconds×speed km/h + speed²/[254×(friction+grade)]. Corrected braking time = speed km/h/(3.6×entered deceleration m/s²). Model input requirements: Friction plus signed grade must be positive.