From what I've gathered, police can ask for all kinds of things that they legally have no right to (searches, being let in, etc), but you can still ignorantly comply. They can even legally lie about shit to persuade you.
You haven't committed any crime. Who's supposed to be serving who here?
Traffic violations are violations of the law and thus are criminal matters (though generally misdemeanors).
You can tell the difference by whether there is a County/State/Federal prosecutor involved. Prosecutors don't bring civil claims to court.
First sentence reads: "In the United States, most traffic laws are codified in a variety of state, county and municipal ordinances, with most minor violations classified as civil infractions."