All of them.
Though often reported as scientific fact, the traffic projections engineers use to justify highway expansions and transportation infrastructure spending are wrong. A new book from Strong Towns, "Confessions of a Recovering Engineer: Transportation for a Strong Town" (September 2021), addresses this damaging oversight.
Cities spend enormous sums of money doing this work, yet a competent third grader with a straightedge could predict just as well.
Charles Marohn, Founder of Strong Towns, author of "Confessions of a Traffic Engineer: Transportation for a Strong America"
Engineers use several methods to predict traffic patterns and project increases. Some rely on past data and assume existing trends will continue. Others project changes based on estimations of how new development will affect travel patterns. All of them fail to respond to the dynamism that results from thousands of individual choices taking place in real time.
"The reality is that it is widely known within the engineering profession that all traffic models are wrong," writes Marohn, a a professional engineer and land-use planner with decades of experience, in "Confessions of a Recovering Engineer."
Even so, engineers, knowing that it's less expensive to overdesign transportation infrastructure in the first place than it is to fix congested arterials, recommend costly expansions. The SR-520 bridge across Lake Washington, where the Department of Transportation projected massive increases in traffic three times over 17 years, is a prime example; traffic volumes decreased during the entire period.
These false projections provide an illusion of certainty that spurs billions of dollars in spending every year. They create self-fulfilling prophesies, adding to congestion. And instead of making our transportation system safer, more affordable, or faster, these investments actively destroy the wealth and financial stability of the communities they are meant to serve.
The second installment of the Strong Towns book series, "Confessions of a Recovering Engineer: Transportation for a Strong Town," challenges the standard practices that have made our transportation system too costly—in time, in money, and in physical safety.
Written by a "recovering engineer" for professional and enthusiast audiences, Marohn's newest book demands accountability and urges a new, bottom-up approach for a transportation system that builds wealth and prosperity for the people who use it.
Strong Towns is a national movement that helps local leaders identify the cause of decline in our cities and take action to build a stronger, financially resilient future. Strong Towns is reshaping the conversation about development in North America with its advocacy for bottom-up, incremental action.
Charles Marohn, President and Founder of Strong Towns
