Design of roads is very simple process all you need to have is
1. Practical Experience
2. Design Standards (AASHTO etc)
For practical design of roads a number of criteria are needed to be considered some of them are given below
1. Design Vehicles (Number of Vehicles passing per day)
2. Design Load
3. Soil Parameters (Load bearing capacity etc)
4. Sub grade characteristic etc
For Detail information please Read Highways and transportation Engineering by Khanna.