Comment by TFNA

This is perhaps a declining problem, as young people in so many countries are growing up as proficient US English speakers due to heavy social media consumption, to the point that US English CAPTCHA challenges would just feel normal. I suspect there are more young people around the world now for whom "crosswalk" is more natural than "zebra" -- in my country, the latter is already a shibboleth that one is an oldie who learned UK English, now regarded here as an antiquated standard after teaching switched to US English.