Page 102 - Highway Engineering Handbook Building and Rehabilitating the Infrastructure

HIGHWAY LOCATION, DESIGN, AND TRAFFIC 85

TABLE 2.3B Minimum Passing Sight
Distance (PSD) for Design Speeds from
20 to 70 mi/h (32 to 113 km/h)
PSD
Design Minimum K-crest
speed, mi/h PSD, ft vertical curve
20 710 180
25 900 289
30 1090 424
35 1280 585
40 1470 772
45 1625 943
50 1835 1203
55 1985 1407
60 2135 1628
65 2285 1865
70 2480 2197
Using S minimum passing sight distance
L length of crest vertical curve
A algebraic difference in grades
(%), absolute value
K rate of vertical curvature
● For a given design speed and an A value,
the calculated length L K A.
● To determine S with a given L and A, use
the following:
For S L: S 52.92 K , where K L/A.
For S L: S 1400/A L/2.
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.
urban areas. To calculate available distance on a crest vertical curve, the driver’s eye
height is 3.5 ft (1.07 m) and the height of the object to be avoided is 2.0 ft (0.61 m).
Where conditions call for the use of a decision sight distance in design that cannot
be achieved, every effort should be made to provide the stopping sight distance values
from Table 2.2. Consideration should also be given to using suitable traffic control
devices to provide advance warning of the unexpected conditions that may be encountered.

2.2.3 Horizontal Alignment and Superelevation
The horizontal alignment of a roadway should be designed to provide motorists with a
facility for driving in a safe and comfortable manner. Adequate stopping sight distance
should be furnished. Also, changes in direction should be accompanied by the use of
curves and superelevation when appropriate in accordance with established guidelines.
Some changes in alignment are slight and may not require curvature. Table 2.5 lists the
maximum deflection angle which may be permitted without the use of a horizontal
curve for each design speed shown. It is assumed that a motorist can easily negotiate
the change in direction and maintain control over the vehicle without leaving the lane.