Sanyodo (Senyodo or Kagetomo-no-michi) can mean:
A region of Honshu along the Seto Inland Sea that was one of the Goki-Shichido (literally, "five provinces and seven circuits") administrative divisions.
A highway that traversed Sanyodo, which was one of the Goki-Shichido.
A route built during the Ritsuryo period.
A route built in the Edo period (Saigoku-kaido).
An abbreviation for "Sanyo Jidoshado Expressway."
See also: List of ryoseikoku
One of the Goki-Shichido, referring to a region of Honshu along the Seto Inland Sea
The region west of Kinai and along the Seto Inland Sea, from present-day West Hyogo Prefecture to Yamaguchi Prefecture, was collectively called Sanyodo.
Mimasaka-no-kuni - separated from Bizen-no-kuni in 713.
The Ritsuryo Period
During the Ritsuryo period the Sanyodo connected Kinai (Naniwa-kyo, Heijo-kyo, and Heian-kyo) and North Kyushu (Dazaifu and Hakata). The Sanyodo was only the first graded highway among the Shichido, and as such it was regarded as the most important because of the expected passage and stays of foreign missions.
The eastern end of the road varied in response to the transfer of capitals such as Heijo-kyo and Heian-kyo. The ancient Sanyodo was approximately six to nine meters wide and generally straight in such a way that Kokufu and other areas throughout the region could be connected efficiently. The Sanyodo stretched northwest from Harima-no-kuni to Mimasaka-no-kuni, which was away from the main route.
The Edo Period
The Edo shogunate improved the Sanyodo as part of its road maintenance policy, which focused on five major highways. There were 42 yadoeki along the highway. During the Edo period, the Sanyodo (otherwise known as Saigoku-kaido) connected Kyoto to Shimonoseki City and further extended to Kokura City by crossing the Kammon-kaikyo Strait from Shimonoseki. This route was part of the truck line linking Edo and Nagasaki City.
The Meiji Period and Onward
The Meiji government started its road management system by assigning numbers to national highways. During the Meiji period, the Sanyodo of the Edo period and earlier became a section of National Highway 3 connecting Tokyo City and Kobe City (separating from National Highway 2 in Kyoto City); and of National Highway 4 connecting Tokyo and Nagasaki City; during the Taisho period it was integrated into National Highway 2 connecting Tokyo and Kagoshima City (separating from National Highway 1 in Mie Prefecture).
The Sanyodo has been passed down to National Highway 2 today, which connects Osaka City and Kita Kyushu City. Its route, however, differs from that of the Ritsuryo and Edo periods in many places. National Highway 2 from Okayama City (Okayama Prefecture) to Fukuyama City (Hiroshima Prefecture) takes a very different route (Kamogata-orai Route).
The Sanyo Jidoshado Expressway and the railway lines of the Sanyo Main Line and the Sanyo Shinkansen Line roughly follow the historical Sanyodo. The Sanyodo has served its function of connecting the Kinki region and Kyushu since the Ritsuryo period, and it continues in that manner today.
When people say "Sanyodo" today, they are generally referring to the Sanyo Jidoshado Highway. The historical Sanyodo and the roads running in parallel with it are collectively called "Old Sanyodo" in Okayama Prefecture and East Hiroshima Prefecture, because National Highway 2 does not follow it between Okayama City (Okayama Prefecture) and Fukuyama City (Hiroshima Prefecture).