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How Traffic Jams Affect Air Quality

No one will be surprised to learn that areas with the largest number of cars on the road see higher levels of air pollution on average. Motor vehicles are one of the largest sources of pollution worldwide. You may be surprised to learn, however, that slower moving traffic emits more pollution than when cars move at freeway speeds. Traffic jams are bad for our air.

Freeway Speed and Air Quality

It seems intuitive that your car burns more fuel the faster you go. But the truth is that your car burns the most fuel while accelerating to get up to speed. Maintaining a constant speed against wind-resistance burns more or less a constant amount. It’s when you find yourself in a sea of orange traffic cones — stuck in what looks more like a parking lot than a highway — that your car really starts eating up gas. The constant acceleration and braking of stop-and-go traffic burns more gas, and therefore pumps more pollutants into the air.

The relationship between driving speed and pollution isn’t perfectly linear, though. One study suggests that emissions start to go up when average freeway speed dips below 45 miles per hour (mph). They also start to go up dramatically as the average speed goes above 65 mph. So, the “golden zone” for fuel-consumption and emissions from your vehicle may be somewhere between 45 and 65 mph.

This leads to a dilemma for urban planners trying to develop roadways that will reduce congestion with an eye to reducing the pollution that it causes. Laying out the traffic cones for massive freeway expansion projects sends air-quality plummeting, but the hope is that air-quality will improve somewhat once the cones are gone and everyone is cruising along happily at regular freeway speeds. Ironically, since the average freeway speeds for non-congested traffic hover around 70 mph and above (with states like Texas looking to increase their speed limits), air-quality is unlikely to improve — and may...
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