Not so much a secret anymore. Apple needs to up their front business game. If they were drug dealers they would have been busted long ago.
But they leaked the project intentionally, do you think they'd have sent cars with multiple big sensors out on the street and expect no one to notice?
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I don't think Apple wants to get in the car building industry, its too volatile. I think they want to get into the car software/interface industry. It's a perfect market for them as most car touch interfaces are terrible.
And yet, they got into the fashion industry...