San'yōdō

is a Japanese term for both an ancient division of the country and the main road running through the region. 
History.
"San'yōdō" was one of the main circuits of the "Gokishichidō" system. It was first established during the Asuka period. Over centuries, the government functions of the "Gokishichido" became less important. However, the traditional eight regions, including "San'yōdō", remained cultural markers.
Geography.
"San'yōdō" is the southern part of southern Honshū. The region is on the Seto Inland Sea. It has includes eight ancient provinces, including
The lands of four prefectures is in the region of "San'yōdō": Hiroshima, Hyōgo, Okayama and Yamaguchi.
The modern national highway, Route 2, the San'yō Expressway, and the San'yō Main Line of the West Japan Railway Company, follow the approximate route of the San'yōdō.