Two of the most important considerations in selecting the route for a proposed highway are 1) the physical features of the area and 2) how these features relate to the geometric design controls. Physical features that affect route selection include topography, ground (soil) conditions, and surrounding land use. Any possible environmental impacts posed by construction of a new highway must also be considered. First, the highway designer reviews topographic, geologic and soil maps as well as available aerial photographs of the area.

The designer looks for conditions that will require sudden changes in alignment. For example, areas that would necessitate connecting long straight sections with sharp curves should be avoided. Areas that are subject to floods or avalanches make highway construction difficult, expensive and/or unsafe.

Highway alignment is influenced by terrain. In general, the terrain or topography of an area is classified as level, rolling or mountainous. In level terrain, selection of an alignment is influenced by factors such as the cost of right-of-way, land use, waterways that may require expensive bridging, existing roads, railroads, and subgrade conditions. In rolling terrain, a number of factors need to be considered, including: grade and curvature, depths of cut and heights of fill, drainage structures, and number of bridges. Grades are the greatest challenge in mountainous country.

              Flat                            Rolling                      Mountainous