What are the advantages of macadam roads?

Tar-macadam is significantly cheaper than cobbles & other block paving types of road surfaces. As the temperature required to melt the tar is around 150C machines needed to lay it need not be complicated or require expensive heat resistant materials.
Compacting vehicles such as rollers can be used through out from gravel foundation to final top finish. Skilled experienced workers
are an advantage & are capable of adjusting construction to cope with different under lying soil types, professional staff are rarely required.

It is also easier to repair both locally as per a pot hole, or by scrapping off only the upper most layers to refurbish the road surface.

Due to its flexibility it can not only resist wear but also reduce the noise of traffic.

The tar coated crushed stone affords greater grip, this can be changed by altering the size of the crushed stone pieces to suit different grip & wear requirements such as exist between junctions, bends, straights or main highways & secondary roads.

The related Asphalt ( tar is replaced by a similar material, but a product of oil refining) brings other advantages such as the addition of rubber from old tires for additional grip, it also be porous to allow rain soak away.

I hope this is useful, helps to start you off & not too late!