No U-turn or left turn sign
R3-18 / Regulatory signs
This sign prohibits both U-turns and left turns at the location.
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Signs to learn alongside this one
Sources & scope
Independent educational summaries by DMV One Way. Sign designs follow MUTCD conventions; state and local rules can add restrictions. Use your current state handbook and the signs on the road for the rules that apply to your trip.
This explanation has not been reviewed yet. Current guide illustrations: public-domain FHWA Standard Highway Signs. Historical illustrations: DMV One Way archive.