Complex sentence: Since the foundation of Edo Tōkaidō was narrowed by the Tokugawa Shogunate, the "westward Tōkaidō" functioned as a seaside road to Kyoto, the "northward Tōkaidō" functioned as a seaside road to Pacific coasts of Tōhoku.
Split into simpler sentences: Since the foundation of Edo Tōkaidō was narrowed by the Tokugawa Shogunate. Thus, the "westward Tōkaidō" functioned as a seaside road to Kyoto, the "northward Tōkaidō" functioned as a seaside road to Pacific coasts of Tōhoku.