> Anyone with a half functioning brain can figure out which ones are the taxis. Unfortunately, there are tons of people with less than half functional brain...

> Anyone with a half functioning brain can figure out which ones are the taxis. Unfortunately, there are tons of people with less than half functional brain... well I'm slightly joking here, but the point is you can't assume your user's cognitive ability. The CAPTCHA as it designed should be obvious to any human that would use your website (including those with impaired cognition) while being hard to robots. The OP is claiming that Google CAPTCHA is an international service and yet can fail to be obvious to a significant portion of non-Americans that would use your website.