Persian Royal Road

The Persian Royal Road was an ancient highway built by the Persian king Darius I in the 5th Century BCE. Darius built the road to facilitate rapid communication throughout his very large empire from Susa to Sardis. These couriers could travel 1,677 miles (2,699 km) in seven days. Most of our knowdelge about the Road comes from the Greek historian Herodotus who wrote, "There is nothing in the world that travels faster than these Persian couriers." Herodotus' praise for these messengers — "Neither snow nor rain nor heat nor darkness of night prevents them from accomplishing the task proposed to them with the very utmost speed" — is the inspiration for the unofficial motto of postal carriers.
Since the time of Herodotus the course of the road has been reconstructed by archeological research, and other historical records. The Road began in the west in Sardis (about 60 miles east of Izmir in present-day Turkey), traveled east through what is now the middle northern section of Turkey to the old Assyrian capital Nineveh (present-day Mosul, Iraq), then traveled south to Babylon (present-day Baghdad, Iraq). From near Babylon, it is believed to have split into two routes, one traveling northwest then west through Ecbatana and on along the Silk Road, the other continuing east through the future Persian capital Susa (in present-day Iran) and then southeast to Persepolis.
Herodotus describes the road between Sardes and Susa as follow (Histories 5.52-53):

Everywhere there are royal stations with excellent resting places, and the whole road runs through country which is inhabited and safe.
Through Lydia and Phrygia there extend twenty stages, amounting to 520 kilometers.
After Phrygia succeeds the river Halys, at which there is a gate which one must needs pass through in order to cross the river, and a strong guard-post is established there.
Then after crossing over into Cappadocia it is by this way twenty-eight stages, being 572 kilometers, to the borders of Cilicia.
On the borders of the Cilicians you will pass through two sets of gates and guard-posts: then after passing through these it is three stages, amounting to 85 kilometers, to journey through Cilicia.
The boundary of Cilicia and Armenia is a navigable river called Euphrates. In Armenia the number of stages with resting-places is fifteen, and 310 kilometers, and there is a guard-post on the way.
Then from Armenia, when one enters the land of Matiene, there are thirty-four stages, amounting to 753 kilometers. Through this land flow four navigable rivers, which can not be crossed but by ferries, first the Tigris, then a second and third called both by the same name, Zabatus, though they are not the same river and do not flow from the same region (for the first-mentioned of them flows from the Armenian land and the other from that of the Matienians), and the fourth of the rivers is called Gyndes [...].
Passing thence into the Cissian land, there are eleven stages, 234 kilometers, to the river Choaspes, which is also a navigable stream; and upon this is built the city of Susa. The number of these stages amounts in all to one hundred and eleven.
This is the number of stages with resting-places, as one goes up from Sardes to Susa. If the royal road has been rightly measured [...] the number of kilometers from Sardes to the palace of Memnon is 2500. So if one travels 30 kilometers each day, some ninety days are spent on the journey.

Because the road did not follow the shortest nor the easiest route between the important cities of the Persian Empire, archeologists believe the western-most 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 (in present-day northern Iran) are coincident with the major trade route known as the Silk Road. However, Darius I made the Royal Road as it is recognized today by improving the road bed and connecting the parts together in a unified whole, primarily as a quick mode of communication using the kingdom's pirradaziš, or messengers. Our information about pirradaziš come from a number of tablets at Persopolis. These tablets refer to the system of horse changing on the Royal road that was called pirradaziš (a word related to modern Persian pishtaz, "post"). From these tablets, we know a lot about the continuation of the road from Susa through the formidable Persian gate to Persepolis -23 stages and a distance of 552 kilometers- and about other main roads in the Achaemenid empire. No less important was, for example, the road that connected Babylon and Ecbatana, which crossed the Royal road near Opis, and continued to the holy city of Zoroastrianism, Rhagae. This road continued to the far east and was later known as Silk road.
Herodotus describes the pirradaziš ,for which he uses another name, in very laudatory words:
There is nothing mortal which accomplishes a journey with more speed than these messengers, so skillfully has this been invented by the Persians. For they say that according to the number of days of which the entire journey consists, so many horses and men are set at intervals, each man and horse appointed for a day's journey. Neither snow nor rain nor heat nor darkness of night prevents them from accomplishing the task proposed to them with the very utmost speed. The first one rides and delivers the message with which he is charged to the second, and the second to the third; and after that it goes through them handed from one to the other, as in the torch race among the Greeks, which they perform for Hephaestus. This kind of running of their horses the Persians call angareion.

The construction of the road as improved by Darius was of such quality that the road continued to be used into Roman times. A bridge at Diyarbakir, Turkey still stands from this period of the road's use. Unfortunatly, the remains of this road will soon go under water as the construction of the Sivand Dam reaches it's last stages. Archaeologists are currently doing their best to save the site by making new discoveries before the watering of the Dam takes place at the end of May.