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Stopping Sight Distance Calculator

Free Stopping sight distance Calculator for civil engineering projects. Enter dimensions to get material lists and cost estimates.

Reviewed by Abdullah, Technical Content Specialist

Reviewed by Abdullah, Technical Content Specialist

Formula

SSD = V × t + V² / (2g(f ± G))

Stopping sight distance equals the reaction distance (speed multiplied by perception-reaction time) plus the braking distance (speed squared divided by twice the gravitational acceleration times the sum of friction coefficient and grade). Positive grade means uphill, negative means downhill.

Worked Examples

Example 1: Highway Design at 60 mph

Problem:Calculate the stopping sight distance for a highway with a 60 mph design speed, 2.5s reaction time, 0% grade, and friction coefficient of 0.35.

Solution:Speed = 60 x 1.467 = 88.0 fps\nReaction distance = 88.0 x 2.5 = 220.0 ft\nBraking distance = 88.0^2 / (2 x 32.2 x 0.35) = 343.7 ft\nSSD = 220.0 + 343.7 = 563.7 ft

Result:Stopping sight distance = 563.7 feet

Example 2: Downhill Road at 45 mph

Problem:Find the SSD on a -4% grade at 45 mph with a friction coefficient of 0.38 and 2.5s reaction time.

Solution:Speed = 45 x 1.467 = 66.0 fps\nReaction distance = 66.0 x 2.5 = 165.0 ft\nBraking distance = 66.0^2 / (2 x 32.2 x (0.38 - 0.04)) = 199.3 ft\nSSD = 165.0 + 199.3 = 364.3 ft

Result:Stopping sight distance = 364.3 feet

Frequently Asked Questions

What is stopping sight distance and why does it matter?

Stopping sight distance (SSD) is the minimum distance a driver needs to see ahead in order to safely stop before hitting an obstacle. It consists of two components: the reaction distance traveled during the driver perception-reaction time, and the braking distance required to decelerate to a complete stop. Highway engineers use SSD to design safe road geometry, determine crest vertical curve lengths, and set speed limits on curves and hills.

How does road grade affect stopping sight distance?

Road grade significantly affects the braking distance component of SSD. On a downhill grade, gravity works against the braking force, increasing the distance needed to stop. On an uphill grade, gravity assists braking and reduces the stopping distance. A positive grade value indicates uphill travel, while a negative value indicates downhill. For example, a 6 percent downgrade can increase braking distance by roughly 30 percent compared to a flat road at the same speed.

References

Reviewed by Abdullah, Technical Content Specialist · Editorial policy