Non-Americans who can't rapidly figure out what's intended from the context.

As I commented elsewhere, sure, when I was first asked I guessed it must be the road markings. But to this day I just select all the road markings that aren't clearly separating lanes.

I don't know if I get loads wrong as a result, but it certainly could catch me out by testing identifying the difference between a 'crosswalk' and whatever other markings you might put across your roads.

To TFA's point, good job reCaptcha, you identified my unfamiliarity with everyday American things.