Tōkaidō (region)

The is a Japanese term for both an ancient division of the country and the main road running through the region. 
History.
"Tōkaidō" 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 "Tōkaidō", remained cultural markers.
The ancient Tōkaidō road was improved and widened in 1604.
It is a geographic region which has 15 ancient provinces, including 
The lands of five prefectures are in the region of "Tōkaidō": Chiba, Ibaraki, Kanagawa, Saitama and Tokyo.
Today, Route 1 is a major national highway that mirrors the route of the old Tōkaidō except in the Tōkyō-Yokohama section.
Route 15 follows the route of the old Tōkaidō between Tōkyō and Yokohama