In this case you did.
You gave them authority to police your behavior when you got your license. It’s all there in the fine print.
Your question also applies to the “authority” of stop lights.
That's duress. Sign this or you can't do anything and I give you almost nothing in return.
Not an enforceable contract under common law, no consideration offered or realistic alternative.
Here, sign this paper or else and I'll fuck your mom. LOL, you signed it, you're my slave now.
The real source of the authority is the gun pointed at your head, but in America we all have them.
So the final source is that most American men are cowards, afraid to die for that freedom they always talk about.
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