Here in Tex. I believe the law is, Move into another lane OR slow down to 20 MPH below the posted speed limit.
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You are correct. I saw some APD officers on 183 a couple of weeks ago, and they'd stop a truck with a squad car. About 150 yards past the truck, there were two motorcycle cops with their radar guns. Any motorist not moving over or slowing to 45 on that road, and they'd be on them like stink on dog poo.