McAdam’s method was simple, yet effective at protecting roadways. He discovered that massive foundations of rock upon rock were unnecessary. Asserting that native soil alone would support the road and traffic upon it, as long as it was covered by a road crust that would protect the soil underneath from water and wear.
This design led to the bitumen-based binding called Tarmacadam. This tar was modified by adding small amounts of Portland cement, resin, and pitch.
Using this method of construction, Nottingham’s Radcliffe Road became the first tarmac road in the world.
In 1909, the Development and Road Improvement Funds Act provided grants to local authorities for approved highways works. The Finance Act 1909-10 based vehicle taxation on the horsepower of the vehicle, remaining in place until 1949, and stated that its revenue would be used for road improvements only.