There was a movie that came out in either the late '70s or early '80s--probably made-for-television--that kind of followed in the footsteps of Duel. A guy in a generic van was going around killing people who made him mad on the road. That was the key difference between this movie and Duel--he stalked more than just one person. And, he'd either put in a tape to signal he was after somebody, or music was just always played on the soundtrack when he was making a kill. Because of this, the movie was always known in the family as "fiddler on the highway." I get absolutely zero hits when searching on this title, however.