<P> 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 . </P> <P> The construction of the road as improved by Darius was of such quality that the road continued to be used until Roman times . A bridge at Diyarbakır, Turkey, still stands from this period of the road's use . The road also helped Persia increase long - distance trade, which reached its peak during the time of Alexander the Great (Alexander III of Macedon). </P> <P> 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 . </P> <P> Euclid is said to have replied to King Ptolemy's request for an easier way of learning mathematics that "there is no Royal Road to geometry," according to Proclus . </P>

What is the approximate distance from susa to persepolis