If Vd is design speed (mph), tp-r is perception-reaction time (sec), a is deceleration rate (ft/sec²) and G is longitudinal grade of the road (%/100). What is the expression for stopping sight distance ?
- ASSD = 1·468 Vd tp-r + Vd² / (30((a/32·2) ± G))Correct
- BSSD = 1·468 Vd tp-r + Vd² / ((a/32·2) ± G)
- CSSD = 1·468 Vd tp-r − Vd² / (30((a/32·2) ± G))
- DSSD = 1·468 Vd tp-r − Vd² / ((a/32·2) ± G)
Solution
Concept
AASHTO stopping sight distance
Formula
SSD = 1.468 V t + V² / [30 (a/32.2 ± G)]
Step-by-step solution
1. SSD = distance during perception–reaction + braking distance. 2. Perception–reaction distance = 1.468 V t (V in mph, 1 mph = 1.468 ft/s). 3. Braking distance = V² / [30 (a/32.2 ± G)].
Explanation
The braking distance is increased on a downgrade (−G in the denominator) and decreased on an upgrade.
Why the other options are wrong
The other expressions omit the factor 30 (with V in mph) or subtract the braking distance.
Shortcut
SSD = 1.468Vt + V²/30(a/32.2 ± G).
Common mistake
Subtracting the braking distance.
Correct answer: A) SSD = 1·468 Vd t_p-r + Vd² / (30((a/32·2) ± G))