Decision Sight Distance

D S D = 1.47 V t (plus maneuver; tables by avoidance maneuver A-E)

Longer than SSD because the driver must detect, decide, and maneuver in a complex environment (exit gore, urban clutter). Handbook tables vs speed and maneuver type. Units: V in mph, t in s → feet with 1.47.

$V$ (mph): Design speed
$t$ (s): Pre-maneuver time

Step-by-step solved example

V=50 mph, t=6.5 s (urban decision, no extra maneuver length). DSD travel?

DSD is a longer detection–decision–maneuver path than SSD.

1. 1.47 Vt
1.47×50×6.5=478 ft.
2. Note
Full DSD tables add avoidance distance; 478 ft is the detection-decision piece.

Answer: ≈ 480 ft of travel during 6.5 s (table DSD is longer)

10 practice questions

0/10 correct

1.DSD is

2.SSD = 1.47 V t_r + V² / (30(a/g±G))

3.1.47 converts

4.Maneuver E (rural avoidance) needs more DSD than

5.V=60 mph, t=2.5 s PRT in SSD: distance=

6.Wet pavement mainly increases

7.Object height / eye height enter

8.Urban clutter argues for using

9.Metric: 0.278 V t with V in km/h gives

10.If t doubles, the 1.47Vt term