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Not really in some countries taxis are specific vehicle types and clearly branded. American ones are ordinary cars with slight modifications.

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"slight modifications" it's entirely

In most places taxis can be identified as: having a specific color/color pattern and/or a ceiling fixture

It is by design because you want taxis to stand out from other vehicles

And in that case the CAPTCHA would still work, as the vehicles are of the same type and have clear branding.

Is 'identify all the similar cars?' a valid capcha though? Surely knowing that something is a taxi is the important part of the test, and a good capcha would through in a branded car that isn't a taxi, which non-Americans are far more likely to fail.

Remember that the goal here is to test that a request isn't coming from a computer, and 'identify all the similar things in these images' is really easy for a computer.

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