What does a round red and white sign with a 'R' and 'R' (or 'RAILROAD CROSSING') inside mean?

RAILROAD CROSSING SIGNS: ADVANCE WARNING SIGN: Round yellow diamond with black 'X' and 'RR' — warns that a railroad crossing is ahead; CROSSBUCK: White 'X' shaped sign with 'RAILROAD CROSSING' that marks the actual crossing location (this is a regulatory sign — must yield to trains); STOP SIGN at crossing: Some crossings have STOP signs requiring all vehicles to stop before the tracks. The combination you're likely to see: advance yellow round sign about 500 feet before → pavement markings (white lines, 'RR' painted) → crossbuck at the crossing. SCHOOL BUSES and TRUCKS carrying placardable hazmat must STOP at every railroad crossing regardless of signals. Other vehicles must yield when a train is present or visible.