Making town standards meet design needs as efficiently as possible is no small task for roadway design. A lot of planning goes into a design, including expected use calculations and simulations for each element in the roadway, such as intersections and merging lanes. We have access to sophisticated CAD programs, enabling us to streamline such calculations and ensure that our designs are that much more thorough.
Another component of roadway design is knowledge of the material science of pavement. Your locale and topographic conditions may require more or less flexible asphalt, a thicker layer of process, etc. This is part of what goes into pavement engineering.
Maintenance and rehabilitation of older pavement is a case in point example of the need for good engineering judgment. Making maintenance repairs with the highest long-term benefit at the lowest possible cost is what you can expect when hiring an engineer.