› Did You Know?

Gentlemen, Stop Your Engines

You spend 2 percent of driving time at traffic lights, but inhale 25 percent of your air pollution there.

You know that smell, when you’re stuck at a red light surrounded by other idling cars? That’s the smell of years bleeding from your life. A new study out of the U.K. confirms this: Although drivers spend just 2 percent of their time at traffic lights, those moments account for 25 percent of their exposure to air pollution.

“Air is not uniformly polluted,” says Prashant Kumar, the study’s lead author and a civil engineer at the University of Surrey. “At intersections, cars stop, idle, then accelerate. It takes time for the particles to disperse.”

Kumar asked drivers to carry pollution monitors while navigating an urban course at different times of the day. He found that the concentration of airborne nanoparticles, which contribute to inflammation and disease in the lungs and circulatory system, was 29 times higher at intersections governed by traffic lights than on the open road. (At 300 nanometers across, nanoparticles are smaller than PM 2.5, which is the typical target in vehicle pollution–monitoring studies.)

Don’t despair—there are ways you can limit your exposure. When possible, choose a route with fewer traffic lights. If they're unavoidable, aim for intersections not surrounded by tall buildings, which prevent the pollutants from dispersing. When you’re sitting at a red light, keep a generous distance from the car in front of you. It will make you look a little crazy and probably draw some exasperated honking, but that’s the cost of cleaner lungs.

How about not driving at all? That would be good for the planet, but it could paradoxically make matters worse for your lungs. The air outside the car—the air that pedestrians and cyclists breathe—is much worse than the air within. The study demonstrated this by showing that drivers who open the windows and turn on the car’s fan expose themselves to between two and four times as much particulate matter as those who keep the windows shut. Joggers, I guess you should start sprinting through the intersections—but try not to breathe hard while doing so (that could make it worse).

Local governments could also help lessen auto-exhaust exposure. Coordinating lights, so that drivers can sail through several at a time rather than stopping and waiting at every intersection, would be good for public health. (I suspect it might lower drivers’ blood pressure, too.) Building overpasses to minimize the number of cars stuck at traffic lights would also reduce exposure.

Finally, Kumar’s study may reignite an old debate: whether we should turn cars off at traffic lights. Idling engines not only expose drivers to pollution but also burn more than 10 billion gallons of gasoline each year. NRDC (disclosure) recommends turning off your car if you will be stopped for more than 30 seconds. The Environmental Defense Fund suggests shutting down for waits of 10 seconds or more. Two auto executives, however, told Car and Driver in 2008 that starting a hot engine is too stressful for most cars. Environmentalists and car manufacturers don’t agree? I can’t believe it.

Of course, you could just buy an electric vehicle or a hybrid that is designed to shut down at traffic lights. Either way, keep the windows shut.

Photo: Oran Viriyincy

onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.

Join Us