What is the ideal soil for road?

Well graded , incompressible if possible, moisture free, and free of any organic materials

none expansive,and have plasticity degree and well grading

Ideal soil for road should have - 

Stability,

Incompressibility,

Permanency of strength,

Minimum changes in volume and stability under adverse conditions of weather and ground water,

Good drainage, and

Ease of compaction

It should be dense,

free from too much moisture,

high in internal friction (abrasion) cohesion

 possess low compressibility and capillary.

Thank you for the invitation but the subject is not within my expertise

Other than properly graded, it all depends on the area your working with some parts of the world need a highly dense soil where others requires a coarser aggergate due to the wet climate to allow for drainage