Pope Francis's visit to Washington, D.C. on September 23, 2015, didn't lead to crippling traffic. AP Photo/Alex Brandon, Pool

As L.A.’s Carmageddon proved a few years ago, congestion alerts can be extremely effective—until they wear off.

The arrival of Pope Francis into Washington, D.C., was supposed to bring with it ungodly traffic jams. But morning rush came and went in the nation’s capital Wednesday without so much as a hint of the predicted Popemageddon. Metro ridership was down 14 percent on a week ago, and Politico reported major traffic jams were “few and far between”:

"Knock on wood, we're doing OK. We're surviving," said Jim Battagliese, the director of traffic and weather operations for WTOP. "This morning was very light, so it looks like people got the message that things were going to be busy and stayed off the roads."

All due respect to Battagliese, Syrian refugees are surviving. D.C. commuters are responding exactly how recent experience in other cities suggested they would. When Los Angeles faced Carmageddon—the complete shutdown of a 10-mile stretch of Interstate 405 for an entire weekend—a few years back, traffic levels plunged along the freeway and nearby roads, according to an analysis by UC transport scholars Brian Taylor and Martin Wachs. L.A.’s sequel, Carmageddon II, failed to live up to the original, but traffic congestion still dropped significantly then, too.

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Carmageddon offered several lessons on predictable traffic jams. One is when Kim Kardashian tweets, people listen. Media hype was a big reason for the congestion decline; Taylor and Wachs write that “disseminating information can also be enormously effective—even more effective than providing alternative travel modes.” Indeed, bus ridership during the first Carmageddon fell, too, despite a bump in service. People stayed home, as per Kim’s orders.

Here’s the thing about traffic: even slight drops or spikes in the number of cars on a road (or, for that matter, slightly poor maneuvers) can have a disproportionate impact on congestion. In the parlance of wonks, traffic is “nonlinear”; as Tom Vanderbilt once explained: “Increase the amount of vehicles by 33 percent and congestion is bound to increase well beyond that.” This is why concepts like congestion pricing work—if just a few people choose other routes or times, then many benefit.

The other big lesson from Carmageddon—and, for that matter, congestion pricing—is that drivers are startlingly quick to detect new traffic patterns. By the time Carmageddon II rolled around, the threats of crippling congestion had worn off a bit, so relatively more people took to the road. In fact, there were “virtually no deviations” from baseline traffic by the end of the second closure. Here’s Taylor and Wachs:

As the event progressed and predictions of Carmageddon failed to materialize, motorists adjusted their behavior in response to more sober media reports and real-time traffic information. Some who had originally planned to avoid driving out of fear of nightmarish congestion likely reverted to their more typical driving behavior by taking highway or arterial detours.

Experts have a name for this, too: induced demand. As more space on a road becomes available, more drivers rush to fill it, sure as the pope’s Catholic. It’s why D.C. might not be out of the woods just yet—and a word to the wise for Philadelphia and New York.

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