Knowledge of the area’s geology allows the highway designer to detect potential problem areas and anticipate subsidence, landslides, and erosion problems. Terrain features are the result of past geologic and climatic processes. Erosion and deposition by running water are major geologic processes in shaping the terrain. A study of the terrain and the character of natural and accelerated erosion can aid in judging the complexity of the erosion and in estimating what erosion control measures may be required.

Some soil types are known to be more erosive than others, and their identification is a valuable aid in route selection and erosion control. The U.S. Department of Agriculture classification of soils is helpful. Soil survey maps, prepared by the (NRCS), show this classification as well as the engineering classification of soils. Local NRCS offices can give much assistance in both soil identification and erosion control measures applicable to the local area.

Problems in route selection for a new roadway can sometimes be avoided. For an existing roadway, however, problems must be recognized and precautions taken in the design.