Once again, Carmaggedon doesn’t materialize; Shutting down half of the I-5 Interstate Bridge over the Columbia River for a week barely caused a ripple in traffic

It’s a teachable moment if we pay attention:  traffic adapts quickly to limits in road capacity

The most favored myth of traffic reporters and highway departments is the notion of traffic diversion:  If you restrict road capacity in any one location, then it will spill over to adjacent streets and create gridlock.  It’s invariably used as an argument against any plans to slow car movement or repurpose capacity for transit, cyclists or people walking.

Time and again, however, when road capacity is reduced, either by design or accident, the predicted gridlock fails to materialize.  Earlier this year, New York closed 14th Street to most car traffic; speeds on parallel streets 13th and 15th, were unaffected, according to traffic monitoring firm Inrix. Similarly, Seattle’s experience with closing the Alaskan Way viaduct (now demolished), and imposing tolls on its replacement, the Highway 99 Tunnel, failed to generate the predicted traffic diversion and gridlock.

Crying Carmaggedon, again

The latest case study of comes from Portland.  Portland is connected to suburbs in Southwest Washington by two Interstate highway bridges that crossing Columbia River. The oldest of the two bridges that carry Interstate 5 over the river , built in 1917, needed to have one of the trunnions that carry the weight of its lift-span replaced, an operation that required the bridge to be closed to traffic for a week. The I-5 bridges usually carry about 120,000 to 130,000 vehicles per day, but that capacity was cut in half as both Northbound and Southbound traffic were diverted to the newer structure.  So instead of 6 lanes, the I-5 freeway was reduced to 3 lanes.

Not surprisingly, the Oregon and Washington Department’s of Transportation predicted massive traffic tie-ups.  The two highway departments predicted four-mile long backups, leading the Vancouver Columbian to forecast that the region would be mired in “trunnion trauma“:

If drivers do not change their travel habits, estimates indicate that backups could stretch for 4 miles on either side of the Columbia River, and the duration of congestion could nearly triple, from seven to 20 hours a day.

Traffic catastrophe “didn’t materialize”

The weeklong closure of the bridge ended Friday September 25, and prompted a story looking back at the region’s experience by The Oregonian’s  transportation beat-reporter Andrew Theen.  He found that despite some occasional slowdowns, traffic between the two states was little different than any other week.


. . . travel patterns largely followed the normal cycle: Evening trips northbound across either of the two bridges were a slog, but that’s typically the case with scores of residents working on the Oregon side of the metro area, even during a pandemic.

Deja vu all over again: A reprise of 1997

Long-time Portland-area residents will know that this is not the first time the bridge has been closed to replace a trunnion. To great fanfare, the bridge was closed for just this purpose in 1997.  The predictions of carpocalypse were even more dire then.  That led local officials to mount a massive PR campaign, to put additional buses on the road, and even arrange for a temporary commuter rail service between Portland and Vancouver.  And then, just as today, the expected gridlock never happened.  Local media were stunned that it was so uneventful.

Back in 1997, a clearly surprised Oregonian team of reporters summed up the experience in a September 16 story: “Gridlock? It’s a breeze for savvy commuters.”

Officials predicted monster traffic jams on the freeways and main arteries in Vancouver and Portland. Total gridlock, most planners said weeks before Tuesday’s closure. But the expected calamity was over before it started, in part because of a media blitz and mass transit options set up by transportation officials.

You’d have thought that this experience would make the two states’ highway departments more reticent to forecast traffic disaster.  Not so.  Local highway engineers gravely warned, things would be worse this time:

This year’s closure likely won’t be so smooth, primarily due to growth in Clark County. Since the September 1997 project, the county’s population has increased by about 175,000.

“I seriously doubt we will see a replay,” said Ryan Lopossa, Vancouver’s streets and transportation manager. “There are just so many people who make that crossing.”

The trunnion experience echoes many other If this gives you a bit of deja vu, dear reader, it should.  Back in January, just before the tunnel opened, the city had to commence demolition of the old viaduct, in order to connect on ramps to the new tunnel.  As a result, the city suddenly lost its old waterfront freeway, and didn’t have access to the new tunnel. State highway officials warned that the city was in for weeks of gridlock.  But when they closed the viaduct, not only did nothing happen, but as we related at City Observatory, traffic on most of downtown Seattle got better. Rather than simply diverting to other city streets, traffic levels went down; as the Seattle Times reported “traffic just disappeared.”

What road closures teach us about travel demand

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 traffic on urban roadways seems 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 or priced, many people can choose 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, when when the capacity goes away or its price goes up, trip making changes to reduce traffic.

If we visualize travel demand as an fixed, irreducible quantity, or an incompressible liquid, it’s easy to imagine that there will be Carmaggedon when a major link of the transportation system goes away.  But in the face of changed transportation system, people change their behavior.  Traffic and congestion is more like a gas, expanding and contracting to fill available space. And while we tend to believe that most people have no choice and when and where they travel, the truth is many people do, and that they respond quickly to changes in the transportation system.  Its a corollary of induced demand:  when we build new capacity in urban roadways, traffic grows quickly to fill it, resulting in more travel and continuing traffic jams. What we have here is “reduced demand”–when we cut the supply of urban road space, traffic volumes fall.

If drivers quickly change their behavior in response traffic capacity, it’s a sign that engineers are crying “Wolf” when they make claims that reducing car capacity of a particular road will produce “gridlock.”  This is 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 often predict.

Carmaggedon never comes

The phenomenon of reduced demand is so common and well-documented that it is simply unremarkable.  Whether it was Los Angeles closing a major section of freeway to replace overpasses, or Atlanta’s I-85 freeway collapse, or the I-35 bridge failure in Minneapolis, or the demolition of San Francisco’s Embarcadero Freeway, we’ve seen that time and again when freeway capacity is abruptly reduced, traffic levels fall as well. There’s a lesson here, if we’re willing to learn it:  if you want to reduce traffic congestion, reduce traffic levels. Whether you do it by restricting capacity, or (more sensibly) by imposing tolls that ask motorist to pay for even a fraction of the cost of the roads they’re using, you get a much more efficient system.