The ancient Romans were the earliest large-scale users of concrete. But after the collapse of the Western Roman Empire, concrete was only used rarely. Concrete as a building material was redeveloped in the mid-18th century. One of the most significant uses of concrete pavement came about in the 1950s with the start of the Interstate Highway System in the US. There are about 45,000 miles of interstate highway in the US. About 60 percent is concrete.