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.