A traveled way on which people, animals, or wheeled vehicles move. The earliest roads developed from the paths and trails of prehistoric peoples; their construction was concurrent with the appearance of wheeled vehicles, which was probably in the area between the Caucasus Mountains and the Persian Gulf sometime before 3000 BC. Road systems were developed that connected the civilizations of Mesopotamia and Egypt and facilitated trade. The first major road was the Persian Royal Road, which extended from the Persian Gulf to the Aegean Sea over a distance of 1,775 miles (2,857 km) and was used from about 3500-300 BC. Originally made for the use of troops and their supplies, were eventually much used by the civilian population for the carriage of goods. This encouraged free trade, helped the advance of civilization, and the subjugation and unification of the tribes. Early roads were about 20 feet wide and had ditches along both sides for drainage purposes. Large stones were laid on the foundation, then smaller ones, or gravel, on top. Traffic and weather blended the road material and helped to form the surface. Stone kerbs were made to hold the road surface together and sometimes a line of stone was laid in the middle too, to help in the binding. The Romans were the first to construct roads scientifically. Their roads were characteristically straight, and the best ones were composed of a graded soil foundation that was topped by four layers: a bedding of sand or mortar; rows of large, flat stones; a thin layer of gravel mixed with lime; and a thin wearing surface of flintlike lava. Roman roads varied in thickness from 3-5 feet, and their design remained the most sophisticated until the modern road-building technology in the late 18th and early 19th centuries. Along the Roman roads were rest houses / mansiones and horse-changing stations / mutationes.