The most economical route would be a straight route, if the ground was just flat, with strong enough soil to support the road, and not obstruction, either man made or natural. That would be the best route.
But unfortunately, reality is far from that. You must overcome a lot of obstacles, not only physical but also even environmental or legal, to build the road.
As a fixed variable, we know we must connect point 1 to point 2, when we plan a road. The rest, depend on many variables, including but is not limited to :
- The topography of the area that your road will pass from. For example you must try to follow the natural ground to the greatest extent possible, without deviating from your direction too much, in order to minimize earth cut and fill amounts
- The amount of traffic that your road must carry (building a 2 lane highway vs 6 lane highway, will affect the route in many locations)
- Any natural obstacles such as hills, mountains, rivers, swamps, forests
- Any man made obstacles, which include any structure we can imagine
- Schedule and budget restrictions
- Environmental concerns
- Zoning and legal concerns
- Geotechnical considerations (the quality (strength) of the soil that will support the road and the traffic above)