1. Stopping Sight Distance
CE 331 Transportation Engineering

2. Sight Distance
Definition: The length of the roadway (or crossroad) ahead that is visible to the driver to enable a vehicle traveling at design speed to
stop before reaching a stationary object in its path
make a decision
Pass
Decide what to do at a RR Xing

3. Stopping Sight Distance (SSD)
SSD = dr + db
dr: distance traveled during the driver’s P-R time
db: distance needed to stop the vehicle from the instant the brake is applied

4. SSD Formula SSD = 1.47Vtr + V2/[30(a/32.2+G)]
Mixed unit equations:
SSD = 1.47Vtr + V2/[30(a/32.2+G)]
SSD: stopping sight distance (ft)
V: vehicle speed (mph)
tr: P-R time, use 2.5sec if not given
a: deceleration rate (ft/s2); g=32.2 fps2
G: grade
SSD = (0.28) tru + 0.u2/[(254)(f+G)]
SSD: stopping sight distance (m)
u: vehicle speed (kph)
f: friction factor (note: f = a/g in appropriate units)

6. Example S = (0.28)t u + u2/[(254)(f+g)] 90 +0.03 1 0.15
Will truck strike tree?
u=90 km/h; t = 1 sec; f= 0.15 3%
170 90
+0.03
S = (0.28)t u + u2/[(254)(f+g)]
= m 1
0.15
YES