How can google’s ReCaptcha verify identity *and* train AI?

I have heard that the reason the image portion of ReCaptcha contains so many driving questions is that Google is using it to train AI for their self-driving cars. But to make it an effective security measure, wouldn’t Google need to verify that the squares the user clicked on contained stop signs or street lights or whatever they asked for? That would mean they already know what is in each image, so how do they gain new information from it? How can they verify the information is correct without knowing beforehand?