Calculate stopping sight distance, design speed, or grade from any two inputs using mph or km/h and ft or m units for road design.

Stopping Sight Distance Calculator

Enter any 2 values to calculate the missing variable

Stopping Sight Distance Formula

The following formula is used to calculate the Stopping Sight Distance.

SSD = 1.47*V*2.5+ (V^2/(30*(0.35+(G/100))))
  • Where SSD is the Stopping Sight Distance (ft)
  • V is the design speed (mph)
  • G is the grade (%) entered as a signed value (positive for upgrades, negative for downgrades). In the formula, (G/100) converts percent to a signed decimal grade.
  • This calculator uses a perception–reaction time of 2.5 seconds and a friction factor of 0.35.

How to Calculate Stopping Sight Distance?

The following example problems outline how to calculate Stopping Sight Distance.

Example Problem #1:

  1. First, determine the design speed (mph).
    • The design speed (mph) is given as: 50.
  2. Next, determine the grade (%).
    • The grade (%) is provided as: +15 (an upgrade).
  3. Finally, calculate the Stopping Sight Distance using the equation above: 

SSD = 1.47*V*2.5+ (V^2/(30*(0.35+(G/100))))

The values given above are inserted into the equation below and the solution is calculated:

SSD = 1.47*50*2.5+ (50^2/(30*(0.35+(15/100)))) = 350.42 (ft)

FAQ

What factors influence the Stopping Sight Distance (SSD)?

Stopping Sight Distance depends on the vehicle speed, the road grade (upgrade vs. downgrade), the driver’s perception–reaction time, the available tire–pavement friction (affected by pavement condition such as wet/icy), and brake performance. In this calculator, speed and grade are inputs, while reaction time (2.5 s) and friction factor (0.35) are treated as fixed assumptions.

How does road grade affect the Stopping Sight Distance?

Road grade changes the effective deceleration along the direction of travel. A downgrade (negative grade) generally increases SSD because gravity acts in the same direction as travel and reduces the net deceleration. An upgrade (positive grade) generally decreases SSD because gravity opposes the motion and increases the net deceleration.

Can the Stopping Sight Distance be reduced?

Yes. Lower speeds, better tire–pavement friction (e.g., dry/high-friction surfaces), and well-maintained braking systems can reduce stopping distance. Driver attentiveness (shorter reaction time) also reduces the reaction-distance portion of SSD.