Stopping Sight Distance

SSD = 1.47 V t + \frac{V ^{2}}{30 ( a / g \pm G )} (V in mph, US)

SSD is perception-reaction distance plus braking distance. AASHTO uses t = 2.5 s and a = 11.2 ft/s² (3.4 m/s²) for design. Grade G is + on upgrades (helps stopping) and − on downgrades. Metric form uses 0.278 Vt + V² / [254(a/g ± G)] with V in km/h.

$V$ (mph or km/h): Design speed
$t$ (s): Perception-reaction time
$G$: Grade (decimal)

Step-by-step solved example

V = 60 mph, t = 2.5 s, a/g = 0.35, G = 0. Find SSD.

SSD = perception-reaction distance + braking distance on the grade.

1. Reaction
1.47×60×2.5 = 220.5 ft.
2. Braking
V² / [30(0.35)] = 3600 / 10.5 = 343 ft. SSD = 564 ft.
$SSD = 564 ft$

Answer: 564 ft

10 practice questions

0/10 correct

1.AASHTO design PRT is

2.Downgrades make SSD

3.Decision sight distance is

4.Braking distance scales with

5.Metric reaction term uses 0.278 because

6.Passing sight distance is used on

7.If a = 11.2 ft/s², a/g ≈

8.SSD must be available along

9.V = 40 mph, t = 2.5 s, G = 0, a/g = 0.35. Reaction distance is

10.Wet pavements are handled in design mainly by