Stopping Sight Distance Calculator (AASHTO Green Book)
Calculate stopping sight distance (SSD) with the AASHTO Green Book formula — brake reaction distance plus braking distance — from design speed, reaction time, deceleration and grade, in metres or feet.
How it works
Level road: SSD = 0.278 × V × t + 0.039 × V² ÷ a (metric: V in km/h, a in m/s², SSD in m) · SSD = 1.47 × V × t + 1.075 × V² ÷ a (US: V in mph, a in ft/s², SSD in ft) · On a grade the braking part becomes V² ÷ [254 × (a ÷ 9.81 + G)] (metric) or V² ÷ [30 × (a ÷ 32.2 + G)] (US), with G the grade as a decimal (uphill +, downhill −).
Stopping sight distance is the length of road a driver must be able to see to stop for an object in the lane. AASHTO's Green Book (A Policy on Geometric Design of Highways and Streets) builds it from two parts: the distance travelled during the 2.5-second brake reaction time, and the braking distance at a deceleration of 3.4 m/s² (11.2 ft/s²), a rate about 90% of drivers exceed. Designers round the result up — 185 m at 100 km/h and 570 ft at 60 mph — and lengthen it on downgrades.
Worked example
At a design speed of 100 km/h with the 2.5 s reaction time and 3.4 m/s² deceleration on a level road: reaction distance = 0.278 × 100 × 2.5 = 69.5 m, braking distance = 0.039 × 100² ÷ 3.4 = 114.7 m, so the stopping sight distance is 184.2 m — 185 m as a design value. The car needs about 10.7 s to come to a stop.
Sources: City of Los Angeles Complete Street Design Manual — E-442 Safe Stopping Distances (AASHTO method) · Stopping sight distance — AASHTO formulas and assumptions
Frequently asked questions
What is stopping sight distance?
It is the distance a driver needs to see ahead to perceive an obstacle, react, and brake to a stop before reaching it. Roads are designed so that at least this much sight distance is available everywhere, including over hill crests and around curves.
Why does AASHTO use a 2.5-second reaction time?
Studies of drivers responding to unexpected objects found that 2.5 s covers about 90% of drivers, including older ones. Many drivers react faster, but design must cover most of the population.
Why 3.4 m/s² and not a harder braking rate?
Most drivers brake at 4.5 m/s² or more in a surprise stop, but 3.4 m/s² (11.2 ft/s²) keeps the car controllable on wet pavement and is comfortably exceeded by about 90% of drivers. Choose a harder rate here to see how much shorter an emergency stop can be.
How does a downgrade change the stopping distance?
Gravity works against the brakes, so the braking part grows. At 100 km/h a −6% grade adds roughly 25 m compared with a level road, while an uphill grade shortens it.
Is stopping sight distance the same as the stopping distance in a driving test?
It is the same idea, but road-design values use conservative assumptions (long reaction time, gentle braking). Driver-handbook stopping distances usually assume faster reactions and harder braking, so they are shorter.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.