“It’s not so much about the measurement being wrong, it’s that the whole underlying thesis is wrong”
— University of Connecticut professor Norman Garrick

Travel Demand Modeling (TDM) for the Ohio River Bridges Project? It was just another example of pseudoscience.

A Twitter denizen named Nolan Gray, who describes himself as a “once and future city planner” offered a valuable link, introducing it with this:

Most city planning and civil engineering heuristics and models—trip generation, floor area ratios, street widths, minimum parking requirements, among dozens of others—are legally-mandated bits of pseudoscience.

And our own ORBP? It’s one of the single most shining, flagrant examples of the perpetual con game.

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.

A long read and a necessary read.

The Broken Algorithm That Poisoned American Transportation, by Aaron Gordon (Vice)

For the last 70 years, American transportation planners have been using the same model to decide what to build. There’s just one problem: it’s often wrong.

In November 2011, the Louisville-Southern Indiana Ohio River Bridges Project published a 595-page document that was supposed to finally end a decades-long battle over a highway. The project was a controversial one, to say the least.

At a time when many cities around the country were re-evaluating whether urban highways had a place in their downtowns, Louisville was doubling down. It not only wanted to keep the infamous “Spaghetti junction” where Interstates 64, 65, and 71 meet in a tangled interchange, but it wanted to build more on top of it. In addition, the political alliance behind the project aimed to expand the I-64 crossing to double the lane capacity, as well as build a whole new bridge just down the river—doubling the number of lanes that crossed the river from six to 12—all for a tidy $2.5 billion.

But in order to get approval to use federal funds for this expensive proposition, the project backers had to provide evidence that Louisville actually needed this expansion. Using a legally-mandated industry practice called Travel Demand Modeling (TDM), the project backers hired an engineering firm to predict what traffic will look like 20 years in the future, in this case, by 2030. They concluded that the number of cross-river trips would increase by 29 percent. The implication was obvious: if they did nothing, traffic would get worse. As a result, the project got federal approval and moved ahead.

Two subsequent studies, however, also funded by the Louisville-Southern Indiana Ohio River Bridges Project, came to a very different conclusion.

Two years later, engineering firm CDM Smith looked at what traffic conditions actually had been while the project was seeking approval. It found that from 2010 to 2013, cross-river traffic had actually fallen by .9 percent.

The other study, this one for potential bond-holders, was far more puzzling. It concluded that by 2030, the combined cross-river traffic would be just 132,000 trips, some 15 percent lower than the SDEIS had predicted. Even worse, according to this new study, the combined 12 lanes of river crossings would carry some 4,000 fewer daily trips than just the I-65 bridge did in 2007 alone, completely undermining the argument that Louisville needed these new bridges.

Aaron Renn, an urban policy researcher and frequent critic of the Ohio River Bridges project, extensively documented these shenanigans. “No matter how crazy this project is,” he wrote back in 2013 when that bond-holder study came out, “it always manages to find ways to show that it’s even more wacky than I thought.”

The project is now finished, and everyone in Louisville can see for themselves which prediction was the better one. In 2018, a post-construction traffic study showed that cross-river trips decreased by 2 percent from 2013 to 2018. As a result, the project has been called by Vox, among others, a “boondoggle” of epic proportions.

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.

Recently, I asked Renn how important those initial, rosy traffic forecasts of double-digit growth were to the boondoggle actually getting built.

“I think it was very important,” Renn said. “Because I don’t believe they could have gotten approval to build the project if they had not had traffic forecasts that said traffic across the river is going to increase substantially. If there isn’t going to be an increase in traffic, how do you justify building two bridges? …”

Jumping to the conclusion, as a rather painful reminder of every meeting I ever attended where the carefully structured “models” were cloaked with unimpeachable “science,” having the effect of reducing rock-headed elected officials to tears of joy at the thought of their own self-deception.

Perhaps the most useful thing the model does is obscure that debate behind a veil of scientific certainty. Behind hard, solid numbers. “From the standpoint of a citizen, these numbers essentially come out of a black box,” he said. “You don’t have any idea how they generated these numbers, so you can’t begin to critique them.”

In other words, the model shuts people up. It may not be honest, but in the world of transportation politics, there’s nothing more valuable than that.