Because the road did not follow the shortest nor the easiest route between the most important cities of the empire, archeologists believe the westernmost sections of the road may have originally been built by the
Assyrian kings, as the road plunges through the heart of their old empire. More eastern segments of the road, identifiable in present-day northern Iran, were not noted by Herodotus, whose view of Persia was that of an Ionian Greek in the West; stretches of the Royal Road across the central plateau of Iran, such as the
Great Khorasan Road, are coincident with the major trade route known as the
Silk Road. However, Darius I improved the existing road network into the Royal Road as it is recognized today. A later improvement by the Romans of a road bed with a hard-packed gravelled surface of 6.25 m width held within a stone curbing was found in a stretch near
Gordium and connecting the parts together in a unified whole stretching some 1677 miles, primarily as a
post road, with a hundred and eleven posting stations maintained with a supply of fresh horses, a quick mode of communication using relays of swift mounted messengers, the kingdom's
pirradazis. In 1961, under a grant from the
American Philosophical Society, S. F. Starr traced the stretch of road from Gordium to Sardis, identifying river crossings by ancient bridge
abutments. ==Legacy==