The Romans built nearly 53,000 miles of roads linking the capital to their far-flung empire. To put this amazing feat in context, consider that the United States, to date, has built 42,000 miles of interstate highways. Roman roads were remarkable for preserving a straight line from point to point, regardless of obstacles. They traversed marshes, lakes, ravines, and mountains.
Roman concrete was, when practical, made of lime mixed with volcanic rock or sand called "pozzolana", named after the place where it was first found, Pozzuoli, near Mount Vesuvius. The pozzolana contained an aluminum silicate from which silica was readily liberated by caustic alkalies, such as calcium hydroxide. Silica combined with the lime to form a solid cementing material that would harden in water. Today, industrial byproducts such as fly ash and silica fume - referred to now as pozzolans - serve to enhance concrete in a similar way.
The Romans are considered to have been the best and most scientific road builders the world has yet seen.