It varies based on state law. The federal DOT can't pass its own traffic laws or regulations for constitutional reasons, so they incentivise states to implement

It varies based on state law. The federal DOT can't pass its own traffic laws or regulations for constitutional reasons, so they incentivise states to implement what they think is a good idea by offering them financial incentives. The best example of this is the 21-year-old drinking age. (They threatened to cut off all highway funding for states that didn't raise it from 18 to 21.) Pretty much every traffic officer I know could find at least 5 reasons to pull over any car, no matter how scrupulous the driver.