Building a Macadam Road — Cheap, Dry Highways from Small Broken Stone

The Romans built superb roads by piling up massive stone foundations, but their method was slow and ruinously expensive. In the 1820s the Scottish surveyor John McAdam threw the foundation away. He realised that a road does not need a heavy base if the soil beneath it is kept dry — and that a surface of small, angular broken stones will lock together under traffic into a hard, smooth crust.

His rules were simple and strict: break every stone small and to a uniform size, raise the road in the middle so water runs off, and keep the subsoil drained. Done right, the stones knit together, the rain sheds away, and the dry ground carries the load. Macadam roads were a fraction of the cost of the old highways and could be built almost anywhere, and they spread across the world.

The method still underlies modern roads. When stone dust alone no longer stood up to fast rubber tyres and their dust, engineers bound the broken stone with tar — tarmacadam, soon shortened to tarmac — and from there to the asphalt that paves the planet. It all begins with small stones and good drainage.