Even where a road sign prohibits U-turns, you may make a U-turn as long as you do not obstruct other traffic.

Explanation

The law says that on sections of road where crossing, U-turns, or reversing are prohibited by road signs, the prohibited action must not be performed. There is no exception for not obstructing other traffic.

Guides that explain this

• Stopping Distance in Japan: Reaction Distance + Braking Distance
• Overtaking and Lane Rules in Japan: Where It Is Banned, and How

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