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Road signs in the United States - Wikipedia
American Highways 101: Visual Guide to U.S. Road Sign …
Manual of Traffic Signs
U.S. Route shield - Wikipedia
US Road Signs: Traffic Sign Encyclopedia - Driving-Tests.org
Road Signs and Meanings (US Traffic Signs List) - DMV Practice Test
Manual on Uniform Traffic Control Devices (MUTCD) - FHWA
Standard Highway Signs (SHS) Publication - FHWA MUTCD - Transportation
Road Signs & Traffic Signs in the US: The Definitive Guide
United States Road Symbol Signs - FHWA MUTCD