To get a driving license in California ... and, I believe, most if not all states ... you have to have a social security number (or swear that you're not entitled to one, perhaps because you're here on a non-working visa, like H-4), and show your passport with a valid I-94.
That's a proper I-94 - the white one - not the green visa waiver one.
And they set the expiry of your driving license to the expiry of your I-94. (Which means I must extend my driving license one of these days, since I've recently got a new, extended I-94.)
Anyway, this means that all those folks from Mexico and further south who technically are breaking the law just by being here ... can't get driving licenses.
So, some percentage of drivers on the road don't have driving licenses. So it goes.
This has a couple of implications. One is that insurance companies don't seem to require that you have a valid US driving license to drive here. Which is interesting. I'm sure if you want money from them, it might be a different story, but at least you can prove you have insurance.
Another thing I read recently is that the LAPD announced they will no longer impound the cars of people who don't have driving licenses but who can't get driving licenses.
That is, if you don't have a driving license when you're stopped, they'll maybe impound your car until you can get one. But if the bureaucracy is such that you can't get one, they'll make an exception, maybe fine you, and let you carry on.
This really gladdened my heart. It's a fantastic example of federalism in action. The federal government has this crazy, insane, dumb, fucking evil law that says "no paperwork, no driving license". Yet local governments can choose to Do The Right Thing. I love it.