Wednesday, March 4, 2015

Induced Traffic Comes in Different Flavors

On several earlier occasions, I’ve written about the phenomenon of induced traffic.  In one post, I used a hypothetical Yosemite campground to illustrate the theory.  Several months before that, I yielded the floor to walkability expert Jeff Speck for a video explanation of induced traffic.

But for those who aren’t yet familiar with the concept, I’ll offer this short explanation.  Drivers have a limited tolerance for congestion.  When they encounter congestion above their tolerance, they find another route, defer their trip to another time, or redirect themselves to a less congested destination.

Therefore, when new traffic capacity is provided, perhaps through the construction of additional travel lanes, drivers who were previously avoiding trips began again to take those trips.  The result is that congestion, after an initial drop, soon returns the same level that it had before the construction, even in the absence of new development creating new trips.

This is a non-intuitive result that conflicts with our instinctive expectation that additional traffic capacity should result in reduced congestion.  But if we seriously consider our own travel decisions, deferring trips when possible to avoid congestion, the validity of the theory soon becomes clear.

This is also where I mention Robert Moses, who grabbed the reins of New York City public works and held them for more than four decades through the middle of the 20th century.  Moses was known for claiming that just one more expressway, freeway, or bridge would make New York City traffic flow freely forever.  He passed away still believing the argument, having failed, despite forty years of contrary examples, to have realized the error in his thinking.

Luckily for us, the lesson that eluded Moses has finally been grasped in the years since his passing.  As I’ve previously written, California is changing its environmental assessment rules to reflect the reality of induced traffic.  And there is a wealth of research and writing spreading the story of induced traffic.

Writing in City Lab, Eric Jaffe offers a simple graph showing the induced traffic effect.  And he notes that the only solutions that can seriously address the phenomenon are transit, transit-oriented development, and congestion pricing.

In Wired, Adam Mann also takes a look at induced traffic, reporting the work of researchers who note that the reverse of induced traffic is also true, that reducing travel lanes doesn’t necessarily cause additional congestion.  (Referring back again to Robert Moses, those who have studied the tussles between Moses and pioneering urbanist Jane Jacobs, may recall their fight over Moses’ plan to run an expressway through Washington Park, claiming that the failure to do so would result in gridlock.  Jacobs won the battle, Washington Park was preserved, and congestion didn’t noticeably change.  It should have been a lesson to Moses, but he wasn’t good at lessons.)

Mann also reports the warning that induced traffic may only be true over a limited range of traffic conditions, although the range likely covers most real world situations.  If a city were to take a two-lane Main Street and convert it to a ten-lane thoroughfare, it’s likely that congestion would be slow to return.  But it’s also likely that the downtown would have been destroyed.

Into this increasing understanding of induced traffic, Adam Millard-Ball, a professor at the University of California, Santa Cruz, notes that, even before considering induced traffic, many of the trip projections for new development may be flawed.

Writing in Access, the transportation journal of the University of California, Millard-Ball reports on his research into the trip generation rates reported by the Institute of Transportation Engineers, the bible on which most traffic studies are based.  He finds that the ITE numbers are consistently and significantly higher than the actual traffic generation results.

He points to several possible causes for the discrepancies.  ITE standards call for only successful projects to be measured, ignoring that a percentage of land development projects fall short of full success and therefore generate less traffic.  Also, the ITE only reports absolute trip numbers when marginal rates would give a better picture in many situations.  As an example, a new grocery store in an underserved portion of town might result in fewer vehicle miles traveled as shoppers can drive fewer miles to serve their needs, but the ITE manual only reports the absolute number of trips.

Combining the growing understanding of induced traffic and the Millard-Ball insights on the ITE numbers, yields a paradoxical view of a possible land development.  A hypothetical developer could be obligated to pay for traffic improvements based on ITE rates.  But when the project stumbles, fewer trips are generated.  However, induced traffic quickly claims the unused new capacity, resulting in congestion being the same as before the project.  The public now complains that city hall failed to require sufficient traffic improvements of the developer, even when the reality is that he was probably docked for too many improvements.

It’s a tangled mess, which only serves to reinforce the need, as noted by Jaffe, for congestion pricing, better transit, and transit-oriented development.

And to add one final twist, most of the analyses look at lane capacity as being the traffic improvement being claimed by induced traffic study.  But other traffic improvements could apparently also be subject to the same phenomenon.

Here in Petaluma, the intersection of E. Washington Street and S. McDowell Boulevard is the busiest intersection in town and is often heavily congested.  There are frequent public calls for improved signal timing, including a recent article in the Argus Courier noting the concerns about the intersection and reporting the comments of City staff on the signal timing options.

I have two comments on the story.  First, I know the City staff quoted and believe that they’re truly doing their best to improve the intersection.  But even more importantly, the traffic signals would be subject to the possibility of induced traffic.  Even in the unlikely event that a change could magically add 20 percent more capacity to the intersection, induced trips would soon claim that capacity, returning traffic congestion to its current level.

There are no easy answers.

Next time, I’ll add another insight to the E. Washington Street and S. McDowell Boulevard intersection challenge, an insight that has broad applicability and tells us much about how we view traffic.

As always, your questions or comments will be appreciated.  Please comment below or email me.  And thanks for reading. - Dave Alden (

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