The AASHTO stopping distance formula is as follows:
s = (0.278 × t × v) + v² / (254 × (f + G))
where:
s – Stopping distance in meters;
t – Perception-reaction time in seconds;
v – Speed of the car in km/h;
G – Grade (slope) of the road, expressed as a decimal. Positive for an uphill grade and negative for a downhill road; and
f – Coefficient of friction between the tires and the road. It is assumed to be 0.7 on a dry road and between 0.3 and 0.4 on a wet road.
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