Re: American imperialism
You beat me to the comment!
Cultural imperialism at its best - plus people unfamiliar with US signage may easily get things wrong - e.g. tiny low quality images on phone - hard to tell if a fragment of a street sign or a bit of something else.
Int being a US person and never driven there, no clue on look and feel of US street signage so useless unless it is an obvious sign (and no I'm not going to research US signage just to pass captchas easily).
Other cultural assumptions I have faced in past captchas include ID the taxi (expecting you to pick yellow cars, and not e.g. black cabs)