When designing a highway there are several factors to consider, the main ones being geometric alignment, material choices, and structural design.
Naturally, one of the most important things to consider when designing a highway is its physical geometry and proportions. To determine the physical layout that a highway will have to have engineers have to consider a number of factors including what the surrounding area of the road is like, how much traffic the road will typically get, the climate of the region, and the expected speed limit on the road. Roads with higher speed limits will be designed very differently from lower speed roadways. Engineers wouldn’t want to put a sharp 90-degree turn on a major highway with a 70mph speed limit as not only could this cause accidents but it is much more likely to become a traffic bottleneck. Consideration is also given to factors such as vision distance and bends or terrains are avoided if possible if they would obstruct a driver’s view. Engineers will always try to give a reasonable stopping distance of visibility when designing highways.
The next topic engineers must ruminate upon when designing a highway is what materials they will choose to make the road surface from. Roads are made of layers that are built upon the soil, which is called the subgrade. The material choice will be affected by the intended traits of the road surface, which is largely determined by the climate of the area. An area that receives heavy snowfall every year will have very different road surfaces from a very dry and warm region.
Maintenance is also a consideration, as some materials require more work in certain climates than others, so reducing maintenance cost is always factored into the material choice. Roads have a foundation layer that is placed on top of the subgrade soil. On top of this foundation layer is the roadbase, which gives the road most of its structural support. Finally, the top layer of the road is referred to as the surface course, and each one of these layers must be constructed out of the optimal material for the loads and climate the road is expected to endure.
In addition to the materials the road will be made out of highway engineers need to determine what the underlying structural design of the roadway will be. To determine this they must begin by looking at what the soil under the road will be like. There are two main structural designs that each has unique advantages and disadvantages. The first road structure is rigid pavement, which as it sounds, is very firm meaning it usually requires very little maintenance. This is because when a load is applied to them they can distribute the force over a large area of the subgrade, meaning rigid pavements can also be built over less sturdy soil. The second road type is flexible pavement, which requires a much more stable subgrade and typically won’t last as long and requires more maintenance. The advantage, however, is that more flexible pavements can expand and contract with changes in the weather without cracking and forming potholes like a more rigid concrete will. Climate, as well as expected loads, will have to be considered when determining the structural design of a new road.