Seattle’s Alaskan Way Viaduct, and why “predicted gridlock almost never happens and what this teaches us about travel demand.”


Not that I’d expect Warren Nash or the Bored with Works to read this, but you should.

Why Carmaggedon never comes (Seattle edition), by Joe Cortright (City Observatory)

Why predicted gridlock almost never happens and what this teaches us about travel demand

Seattle has finally closed its aging Alaskan Way viaduct, a six-lane double-decker freeway that since the 1940s has been a concrete scar separating Seattle’s downtown from Elliott Bay. In a few weeks, much of this capacity will be replaced by a new 3 billion dollar highway tunnel under downtown Seattle, but until then, the city will have to simply do without a big chunk of the highway system that circulates cars around downtown Seattle.

Losing a major freeway that carries nearly hundred thousand vehicles a day through the heart of the city will certainly cause a major disruption to the traffic. The Seattle Times confidently told its readers in early January to prepare for a traffic cataclysm: “the region can’t absorb the viaduct’s 90,000 daily vehicle trips and 30,000 detoured bus riders without traffic jams that likely will ripple out as far as [distant suburbs] Woodinville or Auburn.” Our friends at CityLab echoed the ominous rhetoric with a story headlined “Viadoom: Time for the Seattle Squeeze Traffic Hell.”

That’s pretty scary stuff. But two days into Seattle’s brush with carmaggedon how are things looking?

Fairly good, actually.

It may seem like a stretch to suggest that closing the Alaskan Way viaduct actually made traffic conditions in Seattle better, but in some respects, thats likely to be the case. Worried about getting caught in a traffic jam, it’s likely that many travelers postponed or re-rerouted their trips. If closing the viaduct reduces the number of trips to downtown Seattle, it reduces traffic on other streets as well.

Consequently …

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 100,000 vehicles a day on an urban freeway the Alaskan Way viaduct 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, when when the capacity goes away, so does much of the trip making.

If we visualize travel demand as an fixed, irreducible quantity, 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. 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.