Some Roman innovations still in use today include paved roads, concrete, the Julian calendar and sewer systems. Many Roman inventions were necessitated by the empire's gigantic scale and population.
Through the course of their empire, the Romans built nearly 250,000 miles of roads, extending from Egypt to modern England. At its height, the Roman Empire contained 29 highways extending both in and out of the country. The Roman highway system facilitated trade, but it was primarily intended to help move the empire's armies quickly and efficiently. The Romans' desire to build lasting architectural monuments led to the development of one of the earliest forms of concrete. Known in Latin as "opus caementicium," this concrete was used in the construction of aqueducts, sewers and the Coliseum. The Romans also devised the Julian calendar, which divided the year into 365 days and 12 months. Introduced by Julius Caesar, it was the predominant calendar in Europe and the Americas for centuries, until it was later replaced by the Gregorian calendar in the 16th century. There is only a 0.002 percent difference between the length of the year on both calendars. The Romans also developed sewers in order to transport waste out of cities and avoid contaminating drinking water. At the height of the empire, there were seven major sewer systems running out of Rome.