Sorry, but we’re supposed to render unto Caesar what is Caesar’s and render unto God what is God’s. God is not his employer, God does not make the code of conduct for permissible interactions during a traffic stop. You work for Caesar, you have to follow Caesar’s rules.
This wouldn’t be an easy situation, especially if one had to write a ticket.
But I could think of other ways to witness, like telling someone who had gotten no ticket (or only a warning) “God bless you.” One has to take into account credibility issues as well.