>Those storefront and traffic sign captchas are not useful for training ML models.
Not to get all tin-foil-hat, but this is going to sound like it, but if you have a car that has 9+ cameras upon it that drives in areas full of these, then maybe there would be some use for it for Google.
Bear in mind that I'm not saying that they are doing this but to dismiss it unequivocally as something that can't or wouldn't be done entirely ignores the premise that it could prove useful to other areas of their business, which might have a vested interest in such use (say, for example, if Google or it's parent company were trying to break into the self-driving car area[0]).