In my country (UK) taxis are not yellow either (although yellow is a common colour for taxis throughout the world) but I am still able to guess which pictures are of taxis when the other options are of palm trees and a van.

I think that's the point of these captchas. The average human being has no problem passing the test even if they are not 100% familiar with all the details.

There is obviously a limit and indeed pictures should be chosen to be generic enough to be workable throughout the world, but I feel this is 'problem' is blown out of proportion by some commenters.