“Immigrants” aren’t stupid, and despite what some might think, do have the full ability to engage in normal human behaviors like humor.
Edit: I think some of you are confused about this story. This has nothing to do with the normal road signs that are meant to be used for navigation etc. These are things which are usually blank, and sometimes turned on for information about road construction or things like that: https://www.kbtx.com/2024/01/17/no-joke-feds-are-banning-fun...
Some people in the DOT realized they could put something kind of funny on them to encourage people to do stuff like wear their seatbelts, or not speed, or put messages discouraging drunk driving.