Thoughts on Carmageddon

I’m not talking about the controversial computer game of my childhood, but about the closure of the 405 in Los Angeles for 53 hours. The predicted massive traffic jams failed to materialize, just like every time there’s a closure due to construction, an accident, or an earthquake. The reason is that traffic engineers and planners, the media, and even airlines talk up the possibility of gridlock so much that people choose to stay home or use other modes of transportation. The Huffington Post’s warning that the closure could actually increase carbon emissions because people would take longer detours or cause more traffic jams failed to materialize.

Although normally the induced demand phenomenon is more in the long term than in the short term, if the closure is known to take a very short time, then people can make short-term behavioral changes. They’re not going to move closer to where they work or agitate for better public transit, but they’re going to move non-essential trips to another day, carpool or take transit or bike just the one time, or even sleep one night at the office. Indeed,

Dennis S. Mileti, a sociologist, has spent his career analyzing human behavior around natural and man-made disasters. He advises everyone from the Department of Homeland Security to hazmat workers on how to deliver effective warnings that make people pay attention without panicking and guide them to take precautions and other appropriate actions.

In this case, he said, the message got through because of the blanket of media coverage.

“The public doesn’t change its behavior on its own,” Mileti said. “It behaves on the perceptions formed by the information people are provided.”

Ironically, transit strikes do lead to worse traffic, even in Los Angeles. It’s not because transit is more significant to the population of Southern California than the 405; although more people ride transit in Greater Los Angeles than take the 405 across Sepulveda Pass – about 1.2 million per weekday vs. 500,000 – the transit ridership is much more dispersed, and is dominated by people who can’t afford to drive alone. Instead, the issue is one of media forewarning. Strikes are sudden, and even when they’re threatened, the media focus is never about mitigating the extra traffic, not even in New York.

Potentially, this may be one reason why in the long term, building more roads creates induced demand, and demolishing them causes demand to disappear. Highway openings are widely advertised: politicians love ribbon cutting ceremonies, and the media runs stories about developers and drivers complaining about traffic on parallel roads. Highway closures are more sudden – the two test cases, the Embarcadero Freeway and the West Side Highway, were caused by disaster – but afterward the media coverage and the short-term spike in traffic teach the public that those highways do not exist, leading the traffic to vanish.

Of course, reduced demand isn’t just trips vanishing into thin air. Some trips do get diverted to low-traffic side streets. Others get diverted to mass transit, just as the original construction of the Interstates destroyed transit ridership in the US. Slate has an article about how Carmageddon is teaching the locals that mass transit exists in the first place; it’s possible that it’s going to lead to a long-term increase in ridership, just like short-term spikes in gas prices.

This does not mean long-term road use is going to decline – in fact, it’s going to increase because of the added capacity on the 405. Although the construction of rail transit leads to a reduction in traffic (P.S. if you follow the link, bear in mind its pollution estimates are really low, coming from among other things only counting global warming damage to the US), this assumes business as usual. The main lesson here is not about transit, but about what Los Angeles can expect to happen if an earthquake forces the 405 to be permanently closed, just like the Embarcadero. Although drivers and many business groups are guaranteed to warn about traffic, in reality if such a disaster happens, high traffic will not happen, not in the long run.

Update: as Herbie notes in the comments, St. Louis closed a segment of I-64 for two years, and not only did predicted congestion barely materialize, but also the economic impact of the closure according to business surveys was zero.


  1. Wad

    It’s funny sometimes.

    Carmageddon represented the ultimate dread: A freeway is taken away and people will have gridlock and lose freedom. It turned out, people heeded the message, freeways throughout Southern California were wide open and even the 405 construction was completed a half-day early.

    Now people are free to return to normal — sitting in gridlock.

    • Alon Levy

      It’s a question of routine. Traffic congestion rises or falls to the level that people find acceptable; that’s why building more roads only induces more traffic, rather than making traffic flow more smoothly. At the present cost of driving, enough Angelenos find stop-and-go traffic on the 405 good enough that they’ll keep driving on it; they may collectively oppose congestion and politically demand alternatives, or perhaps more lanes, but they’re not going to individually quit the freeway.

      The familiar has a lot of allure in general. I hesitate to say that traffic was unusually good because people were uncertain about how much of it there would be – for one, I’d need to see numbers about similar reductions in traffic coming from other planned, widely advertised freeway closures – but it’s a good theory to test.

  2. Herbie

    Don’t forget that St. Louis closed 5-miles portions of I-64 for 2 years (140k-170k AADT). Prognastications were made of 3-hour commutes and economic ruin. Well, commutes became only slightly worse, and a survey revealed that the highway closure had no discernible economic impact. It’s an interesting world we live in where highways close and the world moves on while highways open and people prophesy they will create 10s of thousands of jobs and 10s of billions in economic fortunes.

    • David Alexander

      For sampling purposes, was the traffic counts on neighbouring highways and local arterials checked as well? And did Metrolink ridership increase? As a roadgeek, I could easily say that traffic counts on other roads increased, but if the traffic counts AND transit ridership didn’t increase, then it may simply point out the irrelevance of the core in that metropolitan area, something that may be politically impossible to reverse in the short term without some type of massive upheaval in American state and municipal governance.

      • Herbie

        Yes, traffic counts on neighboring highways and local arterials was very closely monitored throughout the project’s timespan. Also, many changes were made, particularly on east-west arterials, to mitigate the expected increase in traffic volumes. E.g. I-70 and I-44 were restriped with an additional lane in each direction and 2-lane portions of Clayton and Ladue roads were converted to 3-lanes and had stop signs replaced with traffic signals.

        My understanding of MetroLink ridership is that it was not significantly affected by the highway closure. People who would have used the closed portions of I-64 would generally not have found MetroLink any easier to access.

      • Wad

        Metrolink reported a 50% increase on the Orange County line, of all places — a line that would have been completely unaffected by a 405 closure. And the prevailing traffic was going into Los Angeles.

        Metrolink did run added service in the San Fernando Valley along the Antelope Valley Line and the Ventura County Line, which normally runs weekday rush hours only. (A handful of Amtrak Pacific Surfliner trains travel north of L.A., providing the corridor with service during off-hours). What ridership was like on these lines isn’t yet known, and it’s hard to discern any forward trends from Carmageddon weekend ridership. (A lot of the riders may have been parents just taking their kids on a train ride for curiosity’s sake.)

        Metro reported increased ridership on the subway and Orange Line. Metro won’t count the ridership bump to traffic totals because the rides were free and there’s no way to gauge a long-term sustained bump in ridership because of Carmageddon.

        Metro also reported slightly higher ridership on Westside routes, but keep in mind that some lines also had free fares and weekend ridership is busy enough that the lines don’t need help.

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