Well my reasoning is that they feature more prominently as "characters" themselves and moving in the foreground as compared to "normal" background cars. Maybe 1.5 stars!
Yes that's sometimes difficult to decide if it counts as an "active" role or not. 1-star/"Background" does not mean no role: it may also just be a "background/scenery" role.
Maybe those deserve 2-star, it is difficult to judge without seeing the movie, and everyone may have different way of rating the roles.