The Speed Sweet Spot

The average driver spends 42 hours sitting in traffic each year, an amount of time that has been climbing steadily since the 2008 recession. Traffic is a huge source of stress and a waste of your precious time, but it’s also an underappreciated source of carbon emissions.

With the exceptions of electric and hybrid vehicles, which switch off their engines in bumper-to-bumper traffic, cars continue to burn fossil fuels and emit carbon dioxide even when they’re not getting us anywhere. Reducing traffic is an underappreciated opportunity to shrink our nation’s carbon footprint.

Researchers have taken a couple of approaches to express fuel waste associated with traffic. The more straightforward way to think of the problem is on an individual driver level—the amount of fuel that a car burns during idling. Passenger cars consume between 0.03 and 0.07 gallons of gasoline for every 10 minutes of idling. A driver who sits in traffic for 42 hours per year, therefore, wastes approximately 12.6 gallons of gas and emits 247 pounds of carbon dioxide equivalents. That doesn’t sound like a lot, but there are more than 200 million licensed drivers in the United States. Do the math, and traffic could be responsible for up to 25 million tons of carbon dioxide emissions.

Traffic scientists take a slightly more complex way of looking at this issue. They prefer to think in terms of raising the average speed of a driving trip, and there are two good reasons for this approach. First, idling cannot be eliminated entirely. There will always be sporting events and concerts that clog roads up, traffic lights, accidents, rubberneckers, double-parkers, inconsiderate jerks who make left turns from the right lane…sorry, I got carried away.

The other reason is that traffic experts have graphed the relationship between average speed and carbon emissions per mile traveled, and there is a very large sweet spot. If you average anywhere between 35 and 65 miles per hour in a trip, the average carbon emissions per mile traveled barely changes at all. Speed demons who average in the 80s or 90s emit more carbon due to engine strain and aerodynamic inefficiencies, while people who average much below 25 miles per hour—usually because they’re caught in stop-and-go traffic—also have very high carbon emissions per mile.

The numbers are dramatic. In an ordinary trip with minimal traffic, you’re likely to emit around 0.7 pounds of carbon dioxide per mile traveled. If your average speed drops to 15 miles per hour, that emissions amount rises to 1.2 pounds. A heavily trafficked trip with an average speed of just 10 miles per hour emits 2 pounds of carbon dioxide per mile traveled—almost triple the emissions of a low-traffic trip.

What does this mean in practice? In 2008, researchers at the University of California, Riverside examined a segment of Interstate 110 in downtown Los Angeles between 4 p.m. and 5 p.m. on a weekday (apparently Angelenos like to take off a little early). Congestion was heavy, with more than half the cars traveling less than 25 miles per hour. The researchers estimated that a congestion traffic mitigation program—ramp metering, congestion pricing, and variable speed limits, for example—could raise the average speed by approximately 20 miles per hour. In addition to saving people time and aggravation, the program would lower carbon dioxide emissions by 12 percent, or 23 tons, during the one-hour period.

Very few people talk about traffic in discussions about climate change. That’s too bad, because cutting congestion on our roadways is essentially a form of energy efficiency—traveling the same distance with less fuel. Addressing traffic is also one of the climate change measures we could take that just might garner bipartisan support: The Capital Beltway, after all, is one of the most heavily congested roads in the country.