I’ve written
often about induced traffic and its near cousin
congestion. But they’re topics for which
the lessons are non-intuitive and therefore bear frequent repetition. There were times in my life when I would have
looked askance at the concept of induced traffic and been in personal need of
recurring lessons. (I’ve learned to be more
humble about my intuition. The world is often
more complex and counterintuitive than my instincts tell me.)
I’ve had conversations
with folks to whom I carefully explained the phenomenon of induced traffic,
including how it makes congestion almost impossible to solve. The other parties nodded knowingly as I spoke,
apparently following the thread of the argument just fine, and then, as their
first question after I finished, asked “So what new roads must we build to solve
congestion?”
Luckily,
there are plenty of folks who are eagerly accepting the challenge of explaining
induced traffic, including the answer that, within practical limits, there are
no roads that will solve congestion. (For the benefit of those in Petaluma, the
answer has significant implications for the on-going Rainier Crossing discussion.) Today, I’ll share the work of a couple of
those folks.
Writing in
Wired magazine, and a veteran of the Los Angeles freeways since early
childhood, Stuart Dee reviews the current academic thinking on induced
traffic, reporting the finding that traffic seems to increase in lockstep with
newly constructed road miles.
As Dee
summarizes the conundrum, “… we humans love moving around. And if you expand
people’s ability to travel, they will do it more, living farther away from
where they work and therefore being forced to drive into town. Making driving easier also means that people
take more trips in the car than they otherwise would.”
Dee’s
solution is one that I’ve previously endorsed, congestion pricing, which is a modulated
approach to a vehicle mileage tax. Drivers, faced with a steep fee to drive
through a particular location at a specific time, let’s say the financial
district of San Francisco during the working day, will look for options to make
their trip at another time. If even just
a few drivers adjust their trips, traffic congestion would ease and the streets
would be more efficiently used for more hours of the day.
Looking at the
same problem but from a different angle, Chuck Marohn of StrongTowns, based in
Brainerd, Minnesota, likens a hierarchical road system, with local
streets feeding onto collectors feeding onto arterials, to a water basin, with brooks
feeding into streams feeding into rivers.
Marohn notes
that downstream flooding is almost always the result of too much water escaping
the upper elevations of the basin and overtopping the riverbanks in the lower
elevations. He then argues that the same
is true of traffic systems, with too many trips congregating on the arterials
where they become congestion.
The current
regulatory approach to flooding is to require strict controls on the outflow from
new development, typically through detention or retention.
Marohn
contends that same approach would address congestion. He argues that we need to build neighborhoods
from which car trips never reach the arterials or in which daily tasks can be
accomplished without cars. In Marohn’s
words, "For nearly seven decades, our national transportation obsession
has been about maximizing the amount that you can drive. We now need to focus on minimizing the amount
you are forced to drive." Luckily,
we know how to do the latter. It’s urbanism.
Two
insightful writers with different life histories and different lifestyles looking
at induced traffic and congestion and reaching similar and complementary
solutions. I use find that a sign of a fundamental
truth, which beats the heck out of flawed intuition.
Before
closing, I should note a related concept I recently came across. At a public meeting of the Sonoma County
Transit Agency, I asked a couple of SCTA planners and traffic engineers about
their thoughts on induced traffic. They
admitted that their task was more the building of roads, but they were nonetheless
well aware of the induced traffic phenomenon in their work.
But rather
than calling it induced traffic, they called it “peak spreading”. Their observation was the drivers who had
time flexibility and an unwillingness to spend time in traffic adjust their
travel times to drive or after the known peak time. As a result, the duration of peak traffic gets
longer over time.
I can
confirm the description. When my wife
and I have a late afternoon appointment in Marin County, we make plans for
dinner and perhaps shopping afterwards, preferring to burn a couple of hours rather
than sitting in traffic. We are induced
not to travel during peak congestion and instead induced to travel at a
slightly off-peak time.
And of
course, the SCTA observation of peak spreading gives a window on the future
when the third lane of 101 is built between Novato and Petaluma. Rather than peak congestion diminishing, the principal
effect will be reduction in its duration.
And then the duration will re-expand as induced traffic takes advantage
of the new lane.
It’s a
painful, but quite real, truth. We’re
not going to build our way out of congestion.
We’ve had a nearly hundred years of trying and it just doesn’t
work. Not only is the failure empirically
evident, but we now have a theoretical basis that confirms the empiricism. And we have solutions, in congestion pricing
and urbanism, to better manage congestion.
Do I think
that all readers now fully grasp induced traffic and the resulting
congestion? Nope, I know that counterintuitive
realities take time to gain a foothold.
But I’ll keep sharing the work of engaging writers like Dee and Marohn
and hope to someday have worn away the granite of flawed intuition.
In my next
post, I’ll muse upon a discussion from a recent hearing I attended, a
discussion that led me to think that too many of us have forgotten what it’s
like to be a kid.
As always,
your questions or comments will be appreciated.
Please comment below or email me.
And thanks for reading. - Dave Alden (davealden53@comcast.net)
The only graphic that I can ever find on the topic: http://transportblog.co.nz/wp-content/uploads/2012/03/induced-demand.jpg
ReplyDeleteLike you, I wish we had an easier way to explain this phenomenon. I like Marohn's analogy a lot.
Great post!
Anon, thanks for the response. And the graphic you link is right on target. Simple and to the point. (For others, Blogspot seems not to allow live links in the comments, so you'll need to cut and paste.)
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