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December 20, 2011

If You Build It, They Will Come

TrafficIn metropolitan areas around the country, it seems like there is always a suite of new highways under construction in the hopes of alleviating congestion.  But, as you sit in traffic on a brand new highway, you may wonder where all those cars around you came from.  Well, you can thank induced travel.

What is induced travel?  It’s the reality that new traffic is generated simply by opening up a new road or expanding an old one.  This happens in two ways.  In the short term, the expanded capacity encourages people to switch from transit to driving, to change the route they choose, or to take new trips altogether.  The new road brings in drivers who were otherwise driven away by the congestion.  In the long run, highway expansions open up the door for sprawl, and as people move farther apart, they are forced to rely more on cars to get around.

The impacts are measurable, too.  Studies indicate that as much as half of new highway capacity is filled with new traffic in just three years.  Further down the road, induced travel can completely fill up all of the new road capacity.  Worse yet, all this new traffic can clog downstream arteries that are not able to handle the growing pulse of vehicles.

Taking induced travel into consideration, why do we continue to build new roads (and neglect the ones we already have) if they don’t reduce congestion, and at the same time they encourage sprawl, deepen our oil addiction, and endanger public health?

The short answer is: money and entrenched interests.  Highway projects continue to dominate transportation funding and spending, even though states can use their transportation dollars flexibly and citizens are clamoring for transportation choices.

That’s the bad news about induced travel, now here’s the good news: Induced travel isn’t just for highway expansion.  Just like a new road, transit has the ability to generate “traffic” through both expanded capacity in the short run and changes in development patterns in the long run.

Better still, transit systems don’t suffer the same congestion problems as highways.  As more drivers take to a highway, congestion builds and the level of service drops for everyone on the road; fewer people get where they’re going in a timely fashion.  The capacity for a train, however, doesn’t drop as more people take it, it just fills up.  A full transit system could lead agencies to expand service, which means more transportation choices for more people.

When planning the future of our transportation infrastructure, it’s important to take induced travel into consideration—building more roads doesn’t eliminate congestion, and it doesn’t help to end our oil addiction, either.    But if we build transportation choices, like transit and walkable, bikeable communities—the people will come.

-- David Loss, Sierra Club Green Transportation Campaign intern

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