Note that in the US, we have strict stop everyone controls at the border (with Canada no less, this isn't just a Mexican border thing), at which you need to show your passport, and we have mobile border controls up to 100 miles within the border.

I used to live near a place where they frequently put the "mobile" control on an interstate in Vermont about 80 miles from the border, so I got stopped by it a lot.

Of course, I've always wondered about how useful the mobile controls are, other than for getting people used to a police state. They are not allowed to ask any US citizens for passports or proof of US citizenship; so they always ask "is everyone in this car a US citizen" and then wave you on if you say yes. When my friend, who is Malaysian, answered "no, I'm not a US citizen", they thought based on his excellent American English accent that he was joking and waved him on anyhow.