I asked this in another thread, or maybe it was another site, but it deserves answering:

What about those of us who do not carry (or have) a driver's license? Obviously, if you're taking the subway, there is a high likelihood that you do not have a car.

Are we going to be asked to carry our US passports on our person? This is going to be interesting.

I do not carry ID, generally, unless I have my briefcase with me where my passport lives.