# 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

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

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

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

Symbol 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
Design Speed 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.