You're driving along, minding your own business, when the window of the car in front of you is suddenly rolled down, and a paper cup is tossed out. What do you do?

Probably you just sigh and ignore it. If you caught up with the offending person and complained, you might get a right to the jaw for your trouble. If you stopped and picked up the cup, someone would probably run into the back of your car, knowing your luck. In sum, it doesn't seem worth it.

But what if you wrote down the license number of the car and reported it to the police?

That's exactly what four police departments in and around Roanoke, Va., have been encouraging for the last two years. And according to Capt. George Hubbard, commander of the patrol division of the Roanoke city police, encouraging citizens to snitch on automotive litterbugs seems to have cut down on the problem.

"It's very hard to tell how much effect this really has," said Hubbard. "People don't report people who don't toss stuff out of cars, so I can't give you statistics. All I can say is that a lot of parents have called in and thanked us. Their kids had the car the night before, and they say they'll talk to the kids about it. It's better to do this than to do nothing."

Hubbard does not arrest anyone on the basis of a littering report. He obtains the home address of the party to whom the offending car is registered, and writes that person a letter.

"We point out that littering is a violation, and we say that if it's observed by a policeman, you will be arrested," Hubbard said. "Yes, I imagine it helps intimidate them a little the next time they're tempted."

Could it work in Washington? "Just like you up in a big city, we have a lot more important things to do than chase litterbugs," Hubbard said. "But I see nothing to be lost."

Neither do I.