Railroad Advance Warning Sign
MUTCD W10-1
A railroad crossing is ahead; slow down and be prepared to stop.
What it looks like
A round yellow disc, the only round warning sign, with a black X and the letters R R marking a railroad crossing ahead.
When you'll see it
In advance of a railroad crossing on both highways and city streets, giving drivers time before they reach the tracks.
What to do
Slow down and be prepared to stop; look and listen for trains, and never start across unless you can clear the tracks completely.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.