The Tokaido is the busiest highway in Japan. In fact, during the Edo period it may be the busiest road in the entire world. It is the main route between eastern and western Japan. There is a constant stream of messengers and goods to and from Edo and the main cities of Western Japan -- Kyoto, Sakai (Osaka) and Nagoya/Gifu.
In addition to the busy official traffic, it is also fairly common for citizens of Edo to make pilgrimages to important religious sites. Most of these sites are in Kamakura or in western Japan. The Tokaido is therefore a busy road at almost all seasons of the year.