To govern an empire stretching from the Indus River to the Mediterranean, King Darius the Great organized the construction of the Royal Road. Stretching approximately 1,600 miles (2,700 km) from Susa in modern-day Iran to Sardis in modern-day Turkey, it was the highway system of the ancient world. The road featured 111 postal stations equipped with fresh horses for royal messengers. While a normal traveler might take 90 days to traverse the distance, the king's couriers could deliver a message from one end to the other in just seven days. This efficiency inspired Herodotus’s famous praise: 'Neither snow nor rain nor heat nor gloom of night stays these couriers from the swift completion of their appointed rounds'—a phrase later adopted by the U.S. Postal Service. The road facilitated not just military movement and taxes, but also the spread of culture and trade across the vast Achaemenid territory.