Roads and transportation
Stopping Sight Distance Calculator
Stopping sight distance (SSD) is the distance a driver needs to see an object, react and brake to a stop. It controls curve design, intersection sight lines and vertical curve lengths.
Formula
AASHTO Green Book: SSD = 1.47Vt + V²/[30(a/32.2 ± G)], with V in mph, t in seconds, a in ft/s² and G as a decimal grade (+ uphill). On level ground this equals 1.47Vt + 1.075V²/a. Design values: t = 2.5 s and a = 11.2 ft/s².
Worked example
60 mph on level ground: reaction = 1.47 × 60 × 2.5 = 220.5 ft; braking = 60² ÷ (30 × 11.2/32.2) = 345.0 ft; SSD = 565.5 ft. The Green Book rounds the design value to 570 ft.
Assumptions and limits
- Wet pavement deceleration of 11.2 ft/s², which most drivers exceed.
- Downgrades lengthen and upgrades shorten braking distance.
- Trucks need longer distances, often offset by higher driver eye height.
Frequently asked questions
What is the AASHTO stopping sight distance at 45 mph?
The Green Book design value is 360 ft (calculated 1.47 × 45 × 2.5 + 1.075 × 45² ÷ 11.2 = 165.4 + 194.4 = 359.8 ft).
Why 2.5 seconds of reaction time?
Research found 2.5 s exceeds the reaction time of about 90% of drivers, including older drivers.
Is stopping sight distance the same as passing sight distance?
No. Passing and decision sight distance are longer and apply to different situations.