Here's a link to an article in the Persian Journal that discusses the Royal Road from Susa to Sardis (today, we'd probably call it something more prosaic like "the S-to-S Highway"). Actually, we can all hope that perhaps someday political conditions in that part of the world might permit the old Royal Road to be restored as a great avenue for tourism, from the shores of the Aegean across the high plains of Anatolia, down through the valley of Mesopotamia and so the foothills of the Zagros Mountains.