Does Traffic Congestion Increase Fuel Consumption and Air Emissions?

By Dom Nozzi

Advocates for better transportation and better quality of life often make an important tactical mistake. And the promoters of car dependency, road widening, and suburban sprawl have achieved an enormous victory.


Because nearly all environmentalists and advocates for a better community quality of life have come to agree with sprawl and road widening advocates that efforts to reduce traffic congestion by, say, widening roads or synchronizing traffic signals, is a common sense way to reduce air pollution and gasoline consumption.

Isn’t this obviously true?

After all, such measures to smooth traffic flow and reduce “stop-and-go” traffic improve fuel efficiency and reduces air emissions. A great many studies confirm this.


Therefore, for decades, environmentalists and quality of life advocates have often joined forces with road widening and sprawl advocates by agreeing that adding turn lanes or travel lanes, or synchronizing traffic signals, is an effective way to reduce fuel consumption and air emissions.

Environmentalists, in other words, continue to oppose road widening because it will promote sprawl, but grudgingly (?) end up admitting to themselves that road widening or traffic synchronization WILL reduce air pollution and gas consumption. Widening a road is not ALL bad.

As a result, the sprawl and road widening lobby has regularly been successful in their efforts to gain political support for widening roads and promoting sprawl. “We need to do it to reduce gas consumption and air emissions!” Most environmentalists, interest groups, and elected officials heartily agree.

There is only one small problem with this “common sense” argument.

It is quite wrong.

In a ground-breaking worldwide study of cities in 1989 (“Cities and Automobile Dependence”), Jeffrey Kenworthy and Peter Newman came to a startling, counterintuitive conclusion: cities that did not spend enormous amounts of money to widen roads and ease traffic flow showed LOWER levels of air emissions and gas consumption than cities which went on a road-widening, ease-of-traffic-flow binge – despite higher levels of congestion.

How could this be?

The reason is that nearly all roads are free to use (there is almost never a need to pay a toll to drive on a road). “Free-to-use” roads inevitably encourage “low-value” car trips. That is, trips that are of relatively low importance, such as a drive across town on a major road during rush hour to rent a video…or buy a cup of coffee.

The most effective way to reduce such “low-value” car trips is to charge motorists for using the road by tolling the road (either with toll booths or electronically). Toll roads are a very equitable “user fee.” The more you use a road, the more you pay. In doing so, motorists are more likely to use the road only for the most important car trips, such as the drive to or from work, or medical emergencies, for example.

When roads are free to use, they become congested quite quickly because of all the “low-value” car trips on the road.

Unfortunately, it is very difficult, politically, to use a toll to charge motorists for using a road. The result is that almost no road is tolled.

Traffic congestion, however, provides a “second best” way to more efficiently use a road by reducing low-value car trips. And congestion occurs without any need for politicians to take wildly unpopular actions.

Traffic congestion shaves off a significant number of low-value car trips by placing a “time tax” on car travel (as brilliantly noted by traffic engineer Ian Lockwood), because when a motorist uses a congested road, they must “pay” with their time, as the congestion will cause delays in their trip.

The result is that a great many motorists find that the congestion is intolerable and decide in both the short- and long-term to do something else. They opt to use a less congested road. They use transit, walk, or bicycle. They travel at non-rush hour times. In the long run, many will move to a location that is closer to their daily destinations as a way to avoid the congestion.

And as Kenworthy and Newman found in their worldwide study of cities, this means that more congested cities see less air pollution and less gas consumption because so many low-value car trips have been eliminated by the congestion.

I should also note that transportation is a zero-sum game: Each time we improve motorist comfort or convenience by widening a road, adding a turn lane, making a road a one-way street, adding more free car parking, or synchronizing traffic signals, we reduce the comfort and convenience of all other forms of travel – transit, walking, and bicycling inevitably become less common because car travel becomes more pleasant, and pleasant car travel makes non-car travel less pleasant and more dangerous. More trips by car – rather than by transit, bicycle, or foot – leads to more gas consumption and air emissions.

Tragically, then, environmentalists and quality of life advocates have ruinously joined forces with the road widening and sprawl advocates to “reduce congestion.” Such efforts, ironically, have led to communities with crushing debt, an awful and downwardly spiraling quality of life, much higher air emissions and fuel consumption, less transportation choice, more car dependency, less civic pride, more sprawl, more highway deaths, and an utterly unlovable city.

It is time to recognize an undeniable truth: The only path to a reduction in car dependence, reduced air emissions, reduced gas consumption, reduced sprawl, more transportation choice, better quality of life, more public health, less traffic injuries and death, less sprawl, a better economic environment, lower taxes, and more civic pride is to take away space from cars by narrowing roads and shrinking parking areas, increasing the cost of car ownership and use, and increasing the inconvenience of driving a car.

Widening roads, adding turn lanes, creating one-way streets, and synchronizing traffic signals are all enormously counterproductive to achieving these essential community objectives.

Traffic congestion, car inconvenience, and higher costs for driving are all friends of a better community, a better quality of life, and a better environment. Don’t continue to let the sprawl and happy car lobby fool you into thinking otherwise.


Visit my urban design website read more about what I have to say on those topics. You can also schedule me to give a speech in your community about transportation and congestion, land use development and sprawl, and improving quality of life.


Or email me at: dom[AT]

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1 Comment

Filed under Bicycling, Economics, Environment, Politics, Road Diet, Sprawl, Suburbia, Urban Design, Walking

One response to “Does Traffic Congestion Increase Fuel Consumption and Air Emissions?

  1. yason

    Also, idling an engine creates less pollution than running the engine with high load. Counterintuitively, it’s better to have a block of road full of cars nearly at idle (i.e. crawling in congestion) than a block of road almost full of cars speeding in free traffic. If we measured the pollution-per-mile only, the latter group would win of course. However, the slow speeds—thanks to congestion—prevent new motorists from entering the road. So the net effect is that the road will be full of cars anyway and given that, it’s better to have their engines underloaded if we’re interested in the absolute amount of exhausted pollution.

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