What sign indicates you are approaching a railroad crossing?

The sign that indicates you are approaching a railroad crossing is characterized by a yellow background featuring a black "X" along with the letters "RR." This sign is specifically designed to alert drivers about an upcoming railroad crossing, emphasizing the need for caution as trains may be nearby. The yellow color is used to capture attention and signify that there is a potential hazard, while the black symbol clearly marks the railroad crossing, ensuring that drivers recognize the specific danger associated with trains.

In contrast, the other options do not serve to indicate railroad crossings. A red sign with a white "X" is typically used for stop signs or no entry signs, which relate to road rules rather than railroad crossings. A blue sign indicating a safety zone is generally associated with emergency services and does not pertain to railroad crossings. Lastly, a green sign showing distance to a crossing would normally indicate the distance to a point of interest, rather than warning of an imminent hazard like a railroad crossing. The distinct design of the yellow "X" sign serves a critical safety function, making it the most appropriate marker for this scenario.