McAdam's road workers checked stone size by putting it in their mouths
John Loudon McAdam insisted the top layer of his roads use stones no bigger than about 20 millimetres. Supervisors carried scales to check, but a labourer had a quicker test: if a stone fit in his mouth, it passed. Small stones mattered because they had to be far narrower than the 100-millimetre iron tyres rolling over them.
McAdam, a Scottish engineer, threw out the idea that a road needed a massive rock foundation. Ordinary soil could carry the traffic, he argued, provided a crust of compacted stone kept water and wear away from it. His roads were nearly flat, rising only three inches from the edge to the centre of a 30-foot carriageway, and raised above the water table so rain ran into side ditches. The cheap result, called macadam, spread to the United States and Australia in the 1820s and across Europe in the following decades.
He built on others. John Metcalf, the first professional road builder of the Industrial Revolution, laid about 180 miles of turnpike in northern England from 1765 and preached drainage and a curved surface. In France, Pierre-Marie-Jérôme Trésaguet described a two-layer method in 1775, large rocks under smaller gravel, recognising that the lower layer's job was spreading weight onto the ground. Thomas Telford dug trenches filled with heavy rock and sloped his roads down from the centre.
Money came from tolls. Early in the 18th century Parliament let local justices put up toll gates in exchange for proper upkeep, and a 1707 act created the first turnpike trust run by people other than justices, on part of the London to Chester road. Across West Africa, meanwhile, the Ashanti kept a network of well-maintained roads, and Dahomey collected yearly tolls based on what travellers carried.
Macadam suited horses but turned dusty and washed out. Edgar Purnell Hooley noticed that tar spilled on a road kept the dust down and left a smooth finish, and in 1901 he patented tarmac, tar and stone mixed by machine before laying and pressed flat with a steamroller.
Source: Road transport
