San'yōdō(山陽道?, literally, "southern mountain circuit" or "southern mountain region") is a Japanese geographical term. It means both an ancient division of the country and the main road running through it. The San'yōdō corresponds for the most part with the modern conception of the San'yō region. This name derives from the idea that the southern side of the central mountain chain running through Honshū was the "sunny" side, while the northern side was the "shady" (山陰 San'in) side.
The San'yōdō, however, continued to be important, and highly trafficked through the Edo period (1603-1867). Running mostly east-west, its eastern terminus, along with those of most of the medieval highways (街道, kaidō), was at Kyoto. From there it ran west through Fushimi, Yodo, Yamazaki, and Hyōgo; from there it followed the coast of the Seto Inland Sea to Hagi, near Shimonoseki, the western terminus of both the San'yōdō and the San'indō, and very near the westernmost end of the island of Honshū. It ran a total of roughly 145 ri (approx. 350 miles).
As might be expected, the road served an important strategic and logistical role in a number of military situations over the course of the years. Emperor Go-Daigo in the 14th century, Toyotomi Hideyoshi in the 16th century, and many others used it to flee from conflict, to return to the core of the country (kinai), or to move troops. Many daimyō also used this road as part of their mandatory journeys (sankin kotai) to Edo under the Tokugawa shogunate. Of course, the road also served the more everyday purpose of providing transport for merchants, traveling entertainers, pilgrims and other commoners.