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.

On the errant notion of downtown two-way gridlock: The "I'll huff and puff and boycott a place I rarely go anyway, except to pass through" approach.

Just imagine if City Hall and captive non-profits like Develop New Albany had devoted just a bit of time educating the populace. Well, parties are more fun, I guess.

On the topic of impending two-way streets, our neighbor Jessica hits the center of the target and drops the mic:

"I'm sorry that this is going to slow down your commute and not allow you to speed down my neighborhood street. I'm sorry that two-way streets are going to make it safer for pedestrians. I'm sorry that this is going to cause less accidents to occur. I'm sorry this is going to make our city a walkable city. I'm sorry that this transition is going to continue to help our local businesses thrive. I'm sorry that this is going to make it safer for my family and all of the families with children that have begun to move to this area. Imagine if your subdivision/neighborhood streets were a speedway and think about how you would feel? Now think about downtown New Albany." 

Her hashtag: #sorrynotsorry

Right on, neighbor. Right on. Yesterday the social media-sphere was filled with the threats of flat-earthers to boycott downtown New Albany because the exact same number of traffic lanes will exist after the two-way reversion, just as they did before, but because the configuration is different, OMIGOD HOW WILL WE EVER MANAGE IT.

Maybe by paying attention?

There is a word that can't be repeated often enough to those who threaten to boycott downtown because two-way streets will require them to drive a tad more slowly and (heaven forbid) pay attention to the community outside their car windows:

Promise?

Even before the mayor has mounted his first self-congratulatory plaque, indignant pass-through drivers are screaming about gridlock, and as usual, Jeff Gillenwater provides the most devastating refutation of these spasms:

If people have to wait for more than a traffic light or two in downtown New Albany after having already sped through the neighborhood, traffic is "backed up" and that's usually just because too many drivers aren't smart enough to take alternate routes all of two blocks in either direction from Spring. It often takes me longer to exit the interstate via the 9th Street ramp in Louisville than it does to navigate the "traffic" in downtown New Albany. It's a lot like parking perception: There are typically so few problems with automobile congestion in Downtown New Albany, any seems like a lot. Those frustrated drivers, though, like to take it out on everybody else once they hit the ramps, driving 70-75 mph and weaving in and out of lanes because everybody knows that it's well worth risking multiple lives to keep them from being five minutes late to wherever they're going. I mean, they might not have time for a second cup of coffee at their destination before they have to get started with their work. If everybody driving a car was 1/3 as important as a lot of them seem to think they are, I could roll down my windows and chat with Nobel Prize winners every morning.

Perhaps my favorite of all was the man who said he'd be taking his business to all the wonderful restaurants on Veterans Parkway.

Dude: don't make promises that you can't keep.

Carmageddon in Atlanta -- or maybe not, because "predictions of terrible traffic in the wake of even major disruptions to the road system are almost never realized."

To hell with facts -- we need more traffic lanes!

Carmaggedon stalks Atlanta, by Joe Cortright (City Observatory)

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

It had all the trappings of a great disaster film: A spectacular blaze last week destroyed a several hundred foot-long section of Interstate 85 in Atlanta. In a city that consistently has some of the worst traffic congestion in the country, losing a key link its freeway system could only mean one thing: Carmageddon ...

 ... The prospect of gridlock makes for great headlines and local TV news stories, but as it turns out, predictions of terrible traffic in the wake of even major disruptions to the road system are almost never realized.

One of the most famous instances of this phenomena was in Los Angeles. In 2011 and 2012, the state highway department closed a 10 mile stretch of Interstate 405 on several weekends to rebuild overpasses. The media was awash in predictions of Carmaggedon. 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 ...