The Louisville highway project is hardly the first time travel demand models have missed the mark. Despite them being a legally required portion of any transportation infrastructure project that gets federal dollars, it is one of urban planning’s worst kept secrets that these models are error-prone at best and fundamentally flawed at worst.

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Travel demand models come in different shapes and sizes. They can cover entire metro regions spanning across state lines or tackle a small stretch of a suburban roadway. And they have gotten more complicated over time. But they are rooted in what’s called the Four Step process, a rough approximation of how humans make decisions about getting from A to B. At the end, the model spits out numbers estimating how many trips there will be along certain routes.

As befits its name, the model goes through four steps in order to arrive at that number. First, it generates a kind of algorithmic map based on expected land use patterns (businesses will generate more trips than homes) and socio-economic factors (for example, high rates of employment will generate more trips than lower ones). Then it will estimate where people will generally be coming from and going to. The third step is to guess how they will get there, and the fourth is to then plot their actual routes, based mostly on travel time. The end result is a number of how many trips there will be in the project area and how long it will take to get around. Engineers and planners will then add a new highway, transit line, bridge, or other travel infrastructure to the model and see how things change. Or they will change the numbers in the first step to account for expected population or employment growth into the future. Often, these numbers are then used by policymakers to justify a given project, whether it’s a highway expansion or a light rail line.

Although there are many reasons the Ohio River Bridges Project was a total urban planning debacle, one that has not gotten much attention is the role travel demand models played in putting lipstick on the $2.5 billion pig. One potential reason for that is because those who work in the field have come to expect nothing less.1

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  To be sure, not everyone who works in the field feels this way. Civil engineers in particular are more likely to defend the models as a useful tool that gets misapplied from time to time. University of Kentucky civil engineering professor Greg Erhardt, who has spent the better part of two decades working on these models, said at their best they are “a check on wishful thinking.” But other experts I spoke to, especially urban planners, tend to view the models as aiding and abetting the wishful thinking that more highways and wider roads will reduce traffic.