Macadam as pioneer of modern roads

John Loudon McAdam (1756-1836), a Scottish engineer, is called the pioneer of modern roads because he introduced a scientific and economical method of road construction in the early 19th century.

His main contributions:

• Roads need no heavy foundation; the natural subgrade, kept dry and well drained, carries the load.
• The road is made of compacted layers of angular broken stone (about 250 mm in total: lower layers of stone up to 50 mm size and a top layer of about 20-25 mm stone), which interlock and bind with their own dust under the compaction of traffic.
• A camber (about 1 in 36) and side drains shed water quickly.
• He showed that the thickness is needed only to protect the subgrade from moisture and to spread the wheel load, since the stress decreases with depth; a massive stone base is not required.

Because of this, roads became cheaper and smoother than the older Telford type. His macadam (later water-bound macadam) is the forerunner of today's bituminous macadam and of the flexible pavements used all over the world.