Macadam roads, developed originally in England and France, are named for the Scottish road builder and engineer, John Louden MacAdam (1756-1836). The term "macadam" originally referred to a road surface made of clean, broken, or crushed ledge stone (from a flat shelf of rock), that was rolled over and flattened by a heavy weight. The gaps between the stones were filled with stone dust and the entire surface was "set" with water.
In the next era of roadbuilding, bituminous material (tar or asphalt) was used. This new type of surface was called bituminous macadam. The terms "plain macadam," "ordinary macadam," or "waterbound macadam" were then used to describe the old type of madacam.
Plain macadam surfaces are almost never constructed now in the United States. This is because they are expensive; in addition, the stones become loosened by the vacuum effect of tires. Many miles of bituminous macadam roads are still in service; however, those roadways tend to be narrow and convex (the center of the surface arches upward). Today's roads that are built to withstand very heavy traffic are usually surfaced with durable portland cement (a mixture of limestone and clay) and reinforced with steel rods.
Sources: Oglesby, Clarkson H., and R. Gary Hicks. Highway Engineering, p. 652; Scott, John S. Dictionary of Civil Engineering, pp. 169, 298; Standard Handbook for Civil Engineers, 3rd ed., pp. 16-40 - 16-41.