4.6 Terrain
Terrain classifications pertain to the general character of a specific route corridor. The terrain classification determines the maximum allowable grades in relation to design speed.
Selection of classification should be chosen on the surrounding terrain of the corridor and not on the roadway profile grade and embankment slopes.
Level or rolling are the two types of terrain often presented when choosing appropriate design criteria since these are the predominant terrains in Texas. Some areas of the El Paso District and some areas of other western Districts may be considered mountainous. When mountainous conditions are encountered, refer to
for appropriate design criteria and design considerations.
4.6.1 Level Terrain
Level terrain is where highway sight distances, as governed by both horizontal and vertical restrictions, are generally long or can be designed to be so without construction difficulties or major expense.
In level terrain, the surrounding terrain is considered to range from 0 percent to 8 percent.
4.6.2 Rolling Terrain
Rolling terrain is where the natural slopes consistently rise above and fall below the road grade and where occasional steep slopes offer some restrictions to horizontal and vertical roadway alignment.
In rolling terrain, the surrounding terrain is generally considered to range from 8.1 percent to 15 percent.
4.6.3 Mountainous Terrain
Mountainous terrain is where longitudinal and transverse changes in the elevation of the terrain, with respect to the road are abrupt, and where benching and side hill excavations are frequently required to obtain acceptable horizontal and vertical alignment.
In mountainous terrain, the surrounding terrain is considered to range over 15 percent.