Abstract
TRAFFIC ENGINEERING THEORY is straightforward: a street is congested because the number of drivers exceeds its capacity. If you enlarge the street, you will eliminate congestion. Unfortunately, seventy-five years of evidence tells us that this almost never happens. Instead, what happens is that the number of drivers quickly increases to match the increased capacity, and congestion returns in full force. It’s called induced demand. These new drivers are the people who were taking transit, carpooling, commuting off-peak, or simply not driving because they didn’t want to be stuck in traffic. When the traffic went away, they changed their habits. Maybe they even moved farther away from work, as the time-cost of their commute went down. Unfortunately, thanks to them and others like them, this honeymoon couldn’t last long.
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Notes
- 1.
Randy Salzman, “Build More Highways, Get More Traffic,” The Daily Progress (December 19, 2010).
- 2.
Ted Chen and Katharine Hafner, “Commute Times Increase One Minute after Freeway Widening Project,” NBC Los Angeles (October 8, 2014), http://www.nbclosangeles.com/news/local/Added-405-Carpool-Lane-Was-it-Worth-the-Delays-278600511.html.
- 3.
Joe Cortright, “Reducing Congestion: Katy Didn’t,” City Observatory (December 16, 2015), http://cityobservatory.org/reducing-congestion-katy-didnt/.
- 4.
Susan Handy, “Increasing Highway Capacity Unlikely to Relieve Traffic Congestion,” UC Davis Institute of Transportation Studies (October 2015), http://cal.streetsblog.org/wp-content/uploads/sites/13/2015/11/10-12-2015-NCST_Brief_InducedTravel_CS6_v3.pdf.
- 5.
Melanie Curry, “Caltrans Admits Building Roads Induces More Driving, But Admitting a Problem Is Just the First Step,” StreetsblogCAL (November 18, 2015), http://cal.streetsblog.org/2015/11/18/caltrans-admits-building-roads-induces-congestion-but-admitting-a-problem-is-just-the-first-step/.
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© 2018 Jeff Speck
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Speck, J. (2018). Understand Induced Demand. In: Walkable City Rules. Island Press, Washington, DC. https://doi.org/10.5822/978-1-61091-899-2_27
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DOI: https://doi.org/10.5822/978-1-61091-899-2_27
Publisher Name: Island Press, Washington, DC
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