Transportation
Stopping Sight Distance
AASHTO-based stopping sight distance calculator for level and graded roadways using design speed, grade, and perception-reaction time.
Transportation · AASHTO
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.
AASHTO Formula (Imperial)
SSD = 1.47 × V × t + V² / [30 × (f + G)]
V = design speed (mph) t = PRT (s)
f = friction factor G = grade (decimal, + = uphill)
Reference: AASHTO A Policy on Geometric Design of Highways and Streets (Green Book), 2018, Chapter 3.
Stopping Sight Distance Calculator
Imperial units — mph, feet
AASHTO range: 15–80 mph
Use negative for downhill — conservative design condition
AASHTO standard: 2.5 s
Results
Stopping Sight Distance—
Reaction Distance—
Braking Distance—
Friction Factor (f) Used—
AASHTO Minimum SSD—
Reference only. All designs must be reviewed and sealed by a licensed PE before use in any document.