Urban highways are often very busy but when they are completely out of commission, it doesn’t necessarily lead to horrific traffic:

You’ll forgive our excessively clinical attitude about this damage—and it’s going to cost tens of millions to fix—but what we have here is a classic “natural experiment” of the kind economists and students of public policy relish. So what happens when we take a major urban freeway out of service for a couple of months? Are Atlanta commuters in for hours of gridlock every day and grisly commutes? Will the region’s economy grind to a halt as a result? We’ll be watching over the next several months to see.

So far, the results are consistent with what we’ve seen in Los Angeles and Minneapolis. Monday morning came, and something funny happened: traffic wasn’t so bad…

So what’s going on here? Arguably, our mental model of traffic is just wrong. We tend to think of traffic volumes, and trip-making generally as inexorable forces of nature. The diurnal flow of 250,000 vehicles a day on an urban freeway like I-85 is just as regular and predictable as the tides. What this misses is that there’s a deep behavioral basis to travel. Human beings will shift their behavior in response to changing circumstances. If road capacity is impaired, many people can decide not to travel, change when they travel, change where they travel, or even change their mode of travel. The fact that Carmageddon almost never comes is powerful evidence of induced demand: people travel on roadways because the capacity is available for their trips, and when the capacity goes away, so does much of the trip making.

If Atlanta can survive for a month or two without a major chunk of its freeway, that’s a powerful indication that more modest steps to alter road capacity don’t really mean the end of the world. If we recognize that traffic will tend to adjust to available capacity, we then end up taking a different view of how to balance transportation against other objectives. For example, this ought to be a signal that road diets, which have been shown to greatly improve safety and encourage walking and cycling, don’t have anything approaching the kinds of adverse effects on travel that highway engineers usually predict.

I do think that this suggests drivers will adjust their behaviors based on what roads are available. At the same time, there is probably a tipping point where reducing too much traffic capacity would make a big difference. This might be especially true in car-driven places like Atlanta and Los Angeles that are known for sprawl. Presumably, places where traffic capacity could be picked up by other transportation options (such as closing the Embarcadero Freeway in San Francisco where driving is already a hassle and other options include BART, Muni, etc.) would fare better. Or, perhaps road capacity has to be reduced gradually so that people have time to adjust and make new choices about travel and where they live and work.

See earlier posts about what happened with Carmageddon I and Carmageddon II in Los Angeles