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 (davealden53@comcast.net)
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