structural:scholar_edition, information:filtering, stylistic:style_transfer
Transportation and communication

Under the Achaemenids, trade was extensive and there was an efficient infrastructure that facilitated the exchange of commodities in the far reaches of the empire. Tariffs on trade, along with agriculture and tribute, were major sources of revenue for the empire.

The satrapies were linked by a 2,500-kilometer highway, the most impressive stretch being the Royal Road from Susa to Sardis, built by command of Darius I. It featured stations and caravanserais at specific intervals. The relays of mounted couriers (the angarium) could reach the remotest of areas in fifteen days. Herodotus observes that "there is nothing in the world that travels faster than these Persian couriers. Neither snow, nor rain, nor heat, nor gloom of night stays these courageous couriers from the swift completion of their appointed rounds." Despite the relative local independence given by the satrapy system, royal inspectors, the "eyes and ears of the king", toured the empire and reported on local conditions.

Another highway of commerce was the Great Khorasan Road, an informal mercantile route that originated in the fertile lowlands of Mesopotamia and snaked through the Zagros highlands, through the Iranian plateau and Afghanistan into the Central Asian regions of Samarkand, Merv and Ferghana, allowing for the construction of frontier cities like Cyropolis. Following Alexander's conquests, this highway allowed for the spread of cultural syncretic fusions like Greco-Buddhism into Central Asia and China, as well as empires like the Kushan, Indo-Greek and Parthian to profit from trade between East and West. This route was greatly rehabilitated and formalized during the Abbasid Caliphate, during which it developed into a major component of the famed Silk Road.