# Stopping Sight Distance

Written by Jerry Ratzlaff on . Posted in Civil Engineering

Stopping sight distance, abbreviated as SSD, is the distance traveled by a vehical from the time it is observer to the time it comes to a stop.

## stopping Sight Distance formulas

 $$\large{ SSD = 1.47 \; V \; t \;+\; 1.075 \; \frac{ V^2 }{ a } }$$ $$\large{ SSD = 1.47 \; V \; t \; \frac{ 1.075 \; V^2 }{ a } }$$ $$\large{ SSD = \frac{ 1.47 \; V \; t \;+\; 1.075 \; V^2 }{ a } }$$

### Where:

 Units English Metric $$\large{ SSD }$$ = sight distance for stopping or passing $$\large{ft}$$ $$\large{m}$$ $$\large{ t }$$ = brake reaction time $$\large{sec}$$ $$\large{s}$$ $$\large{ a }$$ = deceleration rate $$\large{\frac{ft}{sec^2}}$$ $$\large{\frac{m}{s^2}}$$ $$\large{ V }$$ = design speed $$\large{\frac{ft}{sec}}$$ $$\large{\frac{m}{s}}$$ $$\large{ G }$$ = grade percent divided by 100 $$\large{dimensionless}$$

## AASHTO Exhibit 3-1 Stopping Sight Distance

 Design Speed Brake Reaction  Distance Braking Distance  on Level Stopping Sight Distance Calculated Design 15 55.1 21.6 76.7 80 20 73..5 38.4 111.9 115 25 91.9 60.0 151.9 155 30 110.3 86.4 196.7 200 35 128.6 117.6 246.2 250 40 147.0 153.6 300.6 305 45 165.4 194.4 359.8 360 50 183.8 240.0 423.8 425 55 202.1 290.3 492.4 495 60 220.5 345.5 566.0 570 65 238.9 4.505 644.4 645 70 257.3 470.3 727.6 730 75 275.6 539.9 815.5 820 80 294.0 614.3 908.3 910 Note:  Brake reaction distance predicated on a time of 2.5 s; deceleration rate of 11.2 $$ft/s^2$$ used to determine calculated sight distance.