Hey, polluters! This billboard's for you.

A poorly running car engine can spew unburned hydrocarbons out the tailpipe. Although this waste of fuel dirties the air and costs the motorist money, many people don't recognize signs that a tune-up is long overdue.

Colorado scientists now report one successful remedy: gentle warnings from a friendly, personalized billboard.

In the United States, "about 6 percent of the cars are responsible for half the [auto] pollution," notes University of Denver chemist Gary A. Bishop. Finding them can be hard, he says, because most are not old clunkers. Excessive tailpipe emissions from newer cars, which came off the assembly line with efficient engines and pollution controls, reflect poor maintenance, he explains.

Several years ago, Bishop and his University of Denver colleague Donald H. Stedman developed a system that uses infrared light to remotely monitor exhaust from passing cars. They recently decided to offer passing motorists feedback on their emissions of carbon monoxide, which climbs as engine performance deteriorates.

The chemists hooked up their roadside sensors to a novel display. When a car's emissions were in the acceptable range, a sign down the road displayed a smiling car. When carbon monoxide exceeded 1.3 percent of the emission, the displayed car assumed a look of surprise. Vehicles spewing more than 4.5 percent carbon monoxide triggered the billboard's car to frown and its message to warn drivers that the problem is "costing you money!"

The big trick, Bishop says, was keeping track of the moving cars so that the billboard's message always reflected emissions of the approaching vehicle.

During a year of operation, his team now reports, the "Smart Sign" system operated reliably--even when traffic exceeded 1,000 vehicles per hour. It provided some 3 million readings at a cost of 2 cents each. The system gave a poor rating to 1.6 percent of the vehicles.

By photographing license plates, the researchers were able to survey 474 auto owners, including a disproportionately high sampling of those who got fair or poor readings. Three-quarters said that they appreciated the billboard's feedback. Sixteen of the 66 people who had received poor readings said the sign had prompted them to get their car serviced. Another 21 drivers planned to do so.

If this sampling is indicative of the motorists in general, Bishop's team reports in the March 15 ENVIRONMENTAL SCIENCE & TECHNOLOGY, it indicates "that more than 4,000 voluntary repairs were made as a result of Smart Sign readings during the year."

"A creative and thoughtful way to alter behavior ... it's something we certainly would want to consider," says Jason Grumet of the Northeast States for Coordinated Air Use Management, a Boston-based consortium of pollution officials.

"It could be a great tool" for offering motorists feedback between state-mandated emissions tests, agrees Bill Becker, executive director of the State and Territorial Air Pollution Program Administrators in Washington, D.C. However, he notes, automakers are phasing in onboard diagnostics that warn drivers as their engine's performance starts to wane. These could eventually eliminate a need for billboard warnings.