Macadam is a type of road construction pioneered by the Scotsman John Loudon McAdam in around 1820. The method simplified what had been considered state-of-the-art at that point.
He found that 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. He used 2-inch broken stones in a layer 6-10 inches deep and depended on the road traffic to pack it into a dense mass, although for quicker compacting, a cast-iron roller could be used.
With the advent of motor vehicles, dust became a serious problem on macadam roads. The vacuum created under fast-moving vehicles sucks dust from the road surface, creating dust clouds and a gradual raveling (pulling apart) of the road material. This problem was later rectified by spraying tar on the surface to create tar-bound macadam, more commonly known as tarmac.
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