The highway or the road structure must be designed carefully for the traffic load to be carried, physical and geological features and climatic conditions of the locality and various other factors that would affect the stability and utility of the highway roads. For design, the road structure consists of the following four component parts:
1. Subsoil: Subsoil is the natural or prepared soil on which the road has to be formed. It should be stable and robust to safely carry the traffic load and weight of roadway construction.
2. Subgrade: The subgrade or the formation functions as a support to the road surface and its foundation. The life duration of the road primarily depends on a stable and dry subgrade. Its level may be the same or above or below the natural ground level. The support given to the road structure by the subgrade is a crucial factor. So, considerable attention should be paid to the proper preparation of the subgrade before the road structure is laid on it.
3. Base: The base or foundation may consist of two layers, the bottom layer being known as the sub-base or soling or bottoming. The sub-base should be stable and capable of resisting distortion under traffic loads to a great extent. The function of a road base is to transmit the load from the surfacing to the subgrade. The base should possess structural stability and sufficient thickness to develop a good bond with the surfacing.
4. Surfacing: The topmost layer the traffic travels is road surfacing or wearing layer or wearing course. The primary function of road surfacing is to provide a smooth and stable running surfacing that is suitable for the type of vehicle and intensity of traffic anticipated on the road. The surfacing should be impervious and should protect the base and the subgrade from the action of weather and rainwater. The desirable qualities of road surfacing are durability, stability, non-slipperiness, economical and dauntlessness. It should also be able to resist displacement by traffic and should therefore be well keyed on to the base.
The governing factors which would affect the design of the above components of road structure are initial cost, availability of local raw materials, volume and class of traffic, climatic conditions of the locality, etc.