A police car is behind you in traffic. The officer can stop you if:
a. You’re not going as fast as they are.
b. You have committed a violation of law.
c. Your car has a “marijuana rulz” bumper sticker.
d. Just because. They don’t need a reason.
The 4th Amendment prohibits unreasonable search and seizure. The police can stop you for any violation of law, from minor mechanical or equipment violations (and there’s lots of ‘em) to moving violations. Your 1st Amendment Rights to Free Speech and Expression DOES allow you to display the “marijuana rulz” bumper sticker, but it’s liable to attract the attention of the police and that along with OTHER things might create enough for the police to stop you.