Hitching is never entirely safe, and we don't recommend it. Travellers who decide to hitch should understand that they are taking a small but potentially serious risk.
Hitchhikers are a rare sight in Japan, but the practice is not illegal (so long as you don't stop traffic). Some hitchhikers have tales of extraordinary kindness from motorists who have picked them up.
The best places to look for rides are convenience stores and expressway service areas. At any service area look for a free map that shows all the interchanges (IC) and rest stops on the expressway network – important orientation points if you have a limited knowledge of Japanese.