Usui is a mountain pass that lies between Nagano and Gunma Prefectures in Japan. It has served as one of the major transportation routes in central Japan since at least the eighth century. It was featured in numerous Initial D episodes as the home course of Impact Blue. It is also the home course of the real life drift racer Keiichi Tsuchiya.
The pass is perhaps famous for the consecutive arc bridges that lie on the outside of the pass. The corner where these bridges are housed is billed as "C121" in the series.