4.11.2 Decision Sight Distance

Decision sight distance is the distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source, recognize the source, select an appropriate speed and path, and initiate and complete the required maneuver safely and efficiently.
Because decision sight distance gives drivers additional margin for error and affords them sufficient length to maneuver their vehicles at the same or reduced speed rather than to just stop, its values are substantially greater than stopping sight distance.
Table 4-25
shows recommended decision sight distance values for various avoidance maneuvers
Table 4-25: Decision Sight Distance
Design
Speed
(mph)
Avoidance Maneuver Decision Sight Distance (ft)
A
1
B
2
C
3
D
4
E
5
30
220
490
450
535
620
35
275
590
525
625
720
40
330
690
600
715
825
45
395
800
675
800
930
50
465
910
750
890
1030
55
535
1030
865
980
1135
60
610
1150
990
1125
1280
65
695
1275
1050
1220
1365
70
780
1410
1105
1275
1445
75
875
1545
1180
1365
1545
80
970
1685
1260
1455
1650
Notes:
  1. Avoidance Maneuver A: Stop on rural road; t = 3.0s
  2. Avoidance Maneuver B: Stop on urban road; t = 9.1s
  3. Avoidance Maneuver C: Speed/path/direction change on rural road; t varies between 10.2 and 11.2s
  4. Avoidance Maneuver D: Speed/path/direction change on suburban road; t varies between 12.1 and 12.9s
  5. Avoidance Maneuver E: Speed/path/direction change on urban road; t varies between 14.0 and 14.5s t = time in seconds
Examples of situations in which decision sight distance is preferred
 include the following:
  • Interchange and intersection locations where unusual or unexpected maneuvers are required (such as exit ramp gore areas and left-side exits);
  • Changes in cross-section such as lane drops; and
  • Areas of concentrated demand where “visual noise” is present with competing sources of visual information, such as roadway elements, traffic elements, traffic control devices, and advertising signs.
Locations along the roadway where a driver has stopping sight distance but not the extra response time provided by decision sight distance is identified as a
“reduced decision zone.”
 Avoid placement of intersections within a reduced decision zone by relocating the intersection or by changing the grades to reduce the length of the reduced decision zone.