What signs indicate that you are approaching a railroad crossing?

Round yellow signs with a black "X" and flashing lights are specifically used to indicate that you are approaching a railroad crossing. The round shape of the sign signifies caution and alerts drivers to prepare for potential hazards. The black "X" symbolizes the intersection with the railroad tracks and serves as a warning to be vigilant for trains that may be crossing or approaching the roadway. The addition of flashing lights enhances the visibility of the warning, indicating that a train may be nearby. This combination of color and shape is standardized to ensure that all drivers recognize the importance of slowing down and being prepared to stop if necessary when they see these signs.

Other options do not represent railroad crossing signs. Square red signs generally indicate stop or yield information, triangle signs with a yellow background are typically used for warning, and circular green signs provide direction or guidance, not warnings related to railroads. Thus, the correct answer highlights the specific signage meant for railroad crossings to ensure safety for all road users.