When a distinctive mountain peak or saddle is visible along the skyline in the extension of a straight road, such a road is called a yama-ate-dōro (“mountain-aligned road”). These roads are not only excellent in terms of landscape design, but in ancient times—when surveying instruments were rudimentary—they must also have facilitated the construction of straight roads.
In the Nara Basin, the Yoko-oji road, which runs east–west across the southern part of the basin, and the Yamada-michi, which crosses its southernmost area, still remain and can be traced on the Geospatial Information Authority maps. As for north–south routes, there exist the Shimo-tsu-michi, Naka-tsu-michi, and Kami-tsu-michi , which traverse the eastern half of the basin at nearly regular intervals. I have discovered that all of these ancient roads are examples of yama-ate-dōro.
As for the sequence of construction of the ancient roads, it can be inferred that the starting point of the Yok-ooji road was first determined near its western end, by deriving the eastward orientation from the winter solstice sunrise line, and selecting the point where that orientation aligned with Mt. Sotokama-yama. In accordance with the conventional view, the next road to be designed and built was the north–south Shimo-tsu-michi, which intersects the Yoko-oji at right angles and also forms a mountain-aligned line.
Subsequently, from the starting point of the Shimo-tsu-michi on the Yoko-oji, the north–south Naka-tsu-michi and Kami-tsu-michi were laid out at distances of four ri and eight ri respectively (probably measured with a rope). In this process as well, mountains located in orientations perpendicular to the Yoko-oji were sought along the skyline, and the roads were constructed as yama-ate-dōro.
