Transportation
Stopping Sight Distance
AASHTO-based stopping sight distance calculator for level and graded roadways using design speed, grade, and perception-reaction time.
Stopping Sight Distance
Stopping sight distance (SSD) is the minimum distance required for a driver to perceive a hazard, react, and brake to a complete stop. AASHTO defines it as the sum of perception-reaction distance and braking distance.
AASHTO uses a standard perception-reaction time (PRT) of 2.5 seconds, representing the 90th-percentile driver under adverse conditions. The friction coefficient (f) varies by design speed per AASHTO brake testing data. Grade significantly affects braking distance — a downgrade reduces effective friction and is always the conservative condition.
SSD = 1.47 × V × t + V² / [30 × (f + G)]
V = design speed (mph) t = PRT (s)
f = friction factor G = grade (decimal, + = uphill)
At the default inputs — 50 mph design speed, a 3% downgrade, and the AASHTO standard 2.5-second perception-reaction time — the friction factor is f = 0.30, giving a reaction distance of 184 ft and a braking distance of 309 ft, for a total SSD of 492 ft. That exceeds the AASHTO tabulated minimum of 425 ft for 50 mph on level grade — the extra 67 ft is the direct cost of designing for the downgrade rather than a flat roadway.
Stopping Sight Distance Calculator
Imperial units — mph, feet