Hi Jordan,
If you look at Engelbert Kaempfer's detailed description of travelling from Nagasaki to Edo in late 17th century Japan (in my Kaempfer's Japan: Tokugawa Culture Observed, Hawai'i UP, 1999) you'll find relative few stretches a cart with horses could have been used. Moreover, the cost of the upkeep of the highways would increase dramatically considering the damage the wheels would have caused. As it was, some sections between Mishima and Hakone were so easily eroded by heavy rains and usage that they were paved with cobble stones. Very painful in a cart with no springs, as would be other parts of the road. Some sections were so steep that even horses with riders could not manage and those with no right to the use of a palanquin (norimono) were carried up and down mountains in what practically consisted "of no more than the round bottom of a basket, with two handles running up to the height of the small roof" (p. 246).
There was, moreover, the idea that shortening the time required to approach Edo would be dangerous as it would not permit the bakufu sufficient preparations to stop someone considered dangerous. For this reason, only cargo boats were permitted to approach Edo along the sea route. Daimyo processions had to take the slower and less comfortable route by land. For the same reason, a speedier river crossing by boat or the construction of bridges with wide spans that would withstand floods were generally not permitted.
Best,
Beatrice.
Beatrice M. Bodart-Bailey
Honorary
Professor, Australian National University, College of Asia and the Pacific
Professor
emerita, Otsuma Women’s University, Tokyo