But surprisingly, nothing of the kind happened. As Brian Taylor and Martin Wachs explain in an article in Access, people mostly avoided taking trips in the area, or chose alternate routes, with the effect that traffic was actually much lighter than normal. They report that “rather than creating chaos, the first closure greatly reduced traffic congestion.” Taylor and Wachs explain that “crying wolf” about likely gridlock depressed trip-taking in the affected area, but that effect faded as travelers realized things were nowhere as bad as predicted.
But even the year-long closure of I-35W in Minneapolis, following the collapse of a highway bridge over the Mississippi River in 2007 produced similar results. Travelers quickly changed their routes and travel times, and many people simply stopped taking trips that crossed the river. David Levinson reports that there were about 46,000 fewer trips per day across the river after the bridge collapsedLet's add Little Rock to the list. My own fears of a nightmare during the closure of the Broadway Bridge didn't materialize. People chose different routes. They chose different drive times. Some traffic lights were adjusted.
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.