Last week Maine became the fourth state, along with Arkansas, Louisiana, and California, to prohibit smoking in vehicles carrying minors. Its law, which covers all passengers under 16 and allows primary enforcement (meaning that a driver can be pulled over just for violating the smoking ban), is the strictest so far. The Arkansas and Louisiana bans apply only to little children. California's covers anyone below 18, but a smoker can be cited only if he's first pulled over for some other reason. The main backer of Bangor's car smoking ban, after which the state law is modeled, says next year he will push Maine legislators to raise the cutoff age to 18 and increase the penalty, now a $50 fine. Police can't know the age of people in a car until they pull it over, of course, so the law could provide a handy excuse for hassling young adults who smoke, especially if its coverage is extended to 16- and 17-year-olds. Don't like the looks of that long-haired 20-year-old with a NORML bumper sticker? If he's smoking in the presence of someone who might be a teenager, you've got all the justification you need for a traffic stop.