Enter the design speed (mph) and the grade (%) into the Stopping Sight Distance Calculator. The calculator will evaluate and display the Stopping Sight Distance. 

Stopping Sight Distance Formula

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

SSD = 1.47*V*2.5+ (V^2/(30*(.34782+(G/100))))
  • Where SSD is the Stopping Sight Distance (ft)
  • V is the design speed (mph) 
  • G is the grade (%) 

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.
  3. Finally, calculate the Stopping Sight Distance using the equation above: 

SSD = 1.47*V*2.5+ (V^2/(30*(.34782+(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*(.34782+(15/100)))) = 351.15 (ft)

FAQ

What factors influence the Stopping Sight Distance (SSD)?

The Stopping Sight Distance is influenced by several factors including the design speed of the vehicle (V), the grade of the road (%), the driver’s reaction time, the condition of the vehicle’s brakes, and the road conditions (wet, dry, icy). The primary factors in the formula are the design speed and the grade of the road.

How does road grade affect the Stopping Sight Distance?

Road grade, or the slope of the road, significantly affects the Stopping Sight Distance. A steeper grade (either uphill or downhill) can increase the distance required for a vehicle to come to a complete stop. This is because the gravitational force either adds to or subtracts from the braking force, depending on whether the road is sloping downwards or upwards, respectively.

Can the Stopping Sight Distance be reduced?

Yes, the Stopping Sight Distance can be reduced by several methods. Improving the condition of the vehicle’s brakes, reducing the vehicle’s speed, and ensuring the road surface is in good condition can all contribute to a shorter stopping distance. Additionally, driver awareness and reaction times play a crucial role, so driver education and alertness can also help reduce the SSD.