Macadam is a variety of road covering invented in the 19th century. The inventor, John Loudon McAdam, pioneered his technique as a replacement for dirt roads. Even though macadamized roads are rarely used in modern times, its influence is still seen in some forms of road construction.

John McAdam was born in Scotland in 1756, and grew up to working in English road construction efforts around the city of Bristol. After several decades of work, McAdam published two treatises on the need for an improved road system using layered rocks. In 1816, as the surveyor on Bristol Turnpike, McAdam tested out his process for creating roads, called macadamizing.

The original system used for macadam roads involved a triple layer of stone. The bottom two layers were comprised of hand-broken rocks laid to a depth of 8 in (20.3 cm) over a formation level called a subgrade. The top layer was much smaller rocks, made to be no more than 2 in (5 cm) thick. The entire road was then compacted and crushed together by use of an enormous roller. In addition, macadamized roads had a slightly convex shape, so that water would run off into drains on either side, rather than collecting on the road.

The macadam process became popular throughout the world, particularly in the quickly expanding American Northeast. The first American macadamized road was a remake ten-mile section of unpaved road that connected the Maryland towns of Boonsboro and Hagerstown. The Boonsboro Turnpike was completed in 1823, using McAdam’s specific directions. In 1830, work was completed on the 73-mile National Road, which remains one of the only roads in America to still contain macadam sections.

Macadam roads were initially created for use with carriages and horse-powered travel. With the advent of automobiles, the process underwent a variety of changes to meet new challenges posed by the vehicles. Dust thrown up by automobile wheels became a serious problem for travelers, leading to the invention of tar-bound macadam or tarmac. The new process used a layer of tar on the subgrade and bound the layers together during rolling with sand and tar. Many early airports used tarmac pavement around the terminals, leading to the modern usage of the term for the disembarking area around a plane.

Advances in road construction lead to the gradual phase-out of macadam in industrialized nations. Replacements such as concrete and asphalt became popular as technology and synthesized materials became available to aid production. In America, the passage of the 1956 Federal Highways act lead to the modernization of most of the country’s major roads, mostly eliminating macadamized constructions. Some developing nations still use the process, and a few remaining areas of macadam road are protected as historical sites in some American towns.