Page 96 - Highway Engineering Handbook Building and Rehabilitating the Infrastructure

HIGHWAY LOCATION, DESIGN, AND TRAFFIC 79

Stopping Sight Distance. Stopping sight distance is the distance ahead that a motorist
should be able to see so that the vehicle can be brought safely to a stop short of an
obstruction or foreign object on the road. This distance will include the driver’s reaction
or perception distance and the distance traveled while the brakes are being applied.
The total distance traveled varies with the initial speed, the brake reaction time, and the
coefficient of friction for wet pavements and average tires. The values in Table 2.2 were
developed using a reaction time of 2.5 s and a braking deceleration rate of 11.2 ft/s 2
2
(3.4 m/s ). The height of eye was taken as 3.50 ft (1.07 m) and the height of the object as
2.00 ft (0.61 m).
When considering the effect of stopping sight distance, it is necessary to check both
the horizontal and the vertical stopping sight distance. Horizontal sight distance may be
restricted on the inside of horizontal curves by objects such as bridge piers, buildings,
concrete barriers, guiderail, cut slopes, etc. Figure 2.6 shows a diagram describing how
horizontal sight distance is checked along an extended curve. Both formulas and a
nomograph are provided to enable a solution. Many times, where the curve is not long
enough or there are a series of roadway horizontal curves, a plotted-out “graphic” solution
will be required to determine the available horizontal sight distance.

TABLE 2.2 Stopping Sight Distance (SSD) for Design
Speeds from 20 to 70 mi/h (32 to 113 km/h)
Design Design Design Design
speed, mi/h SSD, ft speed, mi/h SSD, ft
20 115 46 375
21 120 47 385
22 130 48 400
23 140 49 415
24 145 50 425
25 155 51 440
26 165 52 455
27 170 53 465
28 180 54 480
29 190 55 495
30 200 56 510
31 210 57 525
32 220 58 540
33 230 59 555
34 240 60 570
35 250 61 585
36 260 62 600
37 270 63 615
38 280 64 630
39 290 65 645
40 305 66 665
41 315 67 680
42 325 68 695
43 340 69 715
44 350 70 730
45 360
Conversions: 1 mi/h 1.609 km/h, 1 ft 0.305 m.
Source: Location and Design Manual, Vol. 1, Roadway Design,
Ohio Department of Transportation, with permission.