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07/15/2011 08:09 pm ET | Updated Sep 14, 2011

'Carmageddon' Could Curb Carbon Dioxide Emissions -- Or Cause Them To Climb

On an average July weekend, 500,000 cars drive on I-405 in the Sepulveda Pass. But this is no average weekend: "Carmageddon" has descended on Los Angeles.

The big question is just how bad it will get -- particularly whether spooked Angelenos will stay off the highways and keep to their own neighborhoods, or perhaps take buses and subways instead. If that happens, maybe this weekend's repair work could help avert climate catastrophe. HuffPost asked experts at University of California, Berkeley for some back-of-the-envelope calculations on what it could all mean for California's carbon emissions.

For every gallon of gas we burn, we put about 19.4 pounds of CO2e (carbon dioxide plus equivalent greenhouse gases) into the air. If all of those 500,000 cars that take the 405 simply stayed off the road altogether, the Berkeley researchers said, that would mean a reduction equivalent to 4.7 million pounds of carbon dioxide. (That figure is based on what Google Maps estimates is the 9.9 mile distance of the closed 405 stretch; it also assumes that people would avoid the road in both directions, although a southbound portion of the 405 will remain open).

But getting the carbon calculation right isn't quite so easy. It's not very likely that all of the people who would have otherwise driven around will instead stay home. Many will take some elaborate detours instead (and they might even expend more carbon dioxide in the process).

Caltrans officials did not respond to a request for details on car ridership estimates for the weekend. Eliot Rose, the deputy director of the Center For Resource Efficient Communities (CREC) at Berkeley, suggested comparing "Carmageddon" to the emergency closure of the San Francisco - Oakland Bay Bridge over Labor Day weekend in 2009. Back then, overall vehicle miles traveled in Bay Area dropped 3.7 percent.

If you compared the size of the Bay Bridge to the 405 in the Sepulveda Pass, Rose said, you would expect a 0.3 percent decrease in vehicle miles traveled in Los Angeles. Do the math and you would then expect a total decrease equivalent to a big-sounding number: 1.6 million pounds of carbon dioxide.

But don't feel too triumphant, green warriors: That number sounds large, but it's only the equivalent of about 1,000 round-trip flights between New York and Los Angeles.

And the math, which has plenty of caveats already, doesn't end there. It's very hard to predict what will actually happen this weekend. Over email, Rose's colleague William Eisenstein, the executive director of the CREC, cautioned that if too many people "take their trips anyway and go by a different route, there is actually a decent likelihood that carbon emissions could go up, not down, as a result of 'Carmageddon.' Presumably when driving, people normally choose the fastest route between two points. If the 405 closure diverts them to longer routes, they might be on the road longer, and hence burn more gas and emit more carbon."

JetBlue is also doing its part to keep carbon climbing: The sold-out flights it's offering this weekend between Burbank and Long Beach will generate about 22 pounds of carbon dioxide per passenger.

This weekend's 405 closure might not be that great of a forecast of a less car-dependent future for Los Angeles, said Ethan Elkind, a climate change research fellow at Berkeley. He has studied a state law designed to curb sprawl, and said that while "it's not too late" for Los Angeles to move away from cars, it's "never going to be the kind of classic layout" of European or even other American cities.

Elkind added that when he used to live in Los Angeles, he frequently found himself stuck on the freeway. "I knew one guy," he said, "who actually brought a portable television with him for traffic on the 405 so he could watch the Lakers game when he got stuck. I hope that's atypical, for safety's sake."