Section 3: Roadside Design Criteria
Anchor: #i1026033Clear Zone
The clear zone distances are shown in Table 8-8.
|
Design Speed (mph) |
Clear Zone Distance (ft) |
|---|---|
|
85 |
80 |
|
90 |
80 |
|
95 |
90 |
|
100 |
100 |
Anchor: #i1026042
Slopes
For safety, 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 8-9. 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.
|
Height of Fill (ft) |
Desirable Slope Rate1 (V:H) |
|
|---|---|---|
|
Terrain |
||
|
Level |
Rolling |
|
|
0-5 |
1:8 |
1:6 |
|
5 and over |
1:6 |
1:6 |
|
Note:
|
||
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 must be sloped at 1V:10H or flatter.
Anchor: #i1026066Medians
The median width is the distance between the inside edge of travel lanes of opposing traffic. Median barriers must be installed when the median widths are less than those shown in Table 8-8.