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Section 3: Roadside Design Criteria

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Horizontal Clearance

The horizontal clearance distances are shown in Table 8-10.

NOTE: Online users can click here to see the below table in PDF format.

Anchor: #i1018196Table 8-10: Horizontal Clearance Distances (US Customary)

Design Speed (mph)

Horizontal Clearance Distance (ft)

85

80

90

80

95

90

100

100

(Metric)

Design Speed (km/h)

Horizontal Clearance Distance (m)

140

24

150

28

160

30



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Slopes

For safety reasons, it is desirable to design relatively flat areas adjacent to the travelway so that out-of-control vehicles are more likely to recover or make a controlled deceleration. Design guide values for the selection of earth fill slope rates in relation to height of fill are shown in Table 8-11. Particularly difficult terrain may require deviation from these general guide values. Where conditions are favorable, it is desirable to use flatter slopes to enhance roadside safety.

NOTE: Online users can click here to see the below table in PDF format.

Anchor: #i1036517Table 8-11 Earth Fill Slope Rates

Height of Fill

Usual Max1 Slope Rate, Vertical:Horizontal

-

Type of Terrain

Flat or Gently Rolling

Rolling

0 - 5 ft [0 - 1.5 m]

1V:8H

1V:6H

5 ft and over [1.5 m and over]

1V:6H

1V:6H

1 Deviation permitted for particularly difficult terrain conditions



The slope adjacent to the shoulder is called the front slope. Ideally, the front slope should be 1V:8H or flatter, although steeper slopes are acceptable in some locations.

The back slope should typically be 1V:6H or flatter. However, the slope ratio of the back slope may vary depending upon the geologic formation encountered. For example, where the roadway alignment traverses through a rock formation area, back slopes are typically much steeper.

The intersections of slope planes in the highway cross section should be well rounded for added safety and increased stability of out-of-control vehicles. Where barrier is placed on side slopes, the area between the roadway and barrier should be sloped at 1V:10H or flatter.

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Medians

The median width is the distance between the inside edge of travel lanes of opposing traffic. Median barriers should be considered when the median widths are less than those shown in Table 8-10.

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