takes advantage of laws meant to help the disabled for personal gain.

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If dogs are colorblind, how can guide dogs know when a traffic light turns green?

They don't.

The human handler is responsible for listening to traffic and knowing from which direction it is coming. Sometimes you can hear a grinding click sound when the lights change, while in some countries there is an audible beep to tell the blind when the light has changed.

If the handler gives a "forward" command and the dog sees or hears a car coming at them, the dog disobeys the "forward" command. So the dog focuses only on the cars, not on traffic lights.