With driving down, there's a growing backlash against unnecessary highways

America's state transportation agencies have always loved building new roads. And for most of the 20th century they had a ready justification: car travel was increasing each and every year, so they needed to keep the asphalt flowing.

Since 2007, however, US vehicle travel has plateaued, thanks to both the recession and possibly due to demographic shifts. Yet many states have kept building new roads and highways at a frenetic pace. They've often rationalized this by putting out questionable traffic projections showing that vehicle travel will soar in the future, even though these projections have proved incorrect year after year so far:

For years, states overestimated how much we drive

The black line shows actual vehicle miles traveled as tracked by the Federal Highway Administration. The colored lines are projections from the US Department of Transportation's aggregation of state forecasts. (Frontier Group)

That gap between forecast and reality has become increasingly untenable. And now we're starting to see a backlash. Around the country, new highway projects based on inflated traffic predictions are coming under closer scrutiny — with some states are starting to rethink things entirely.

Wisconsin used murky traffic forecasts to justify a highway expansion — and got slapped down

Exhibit A: Wisconsin's Department of Transportation had been citing forecasts of ever-growing traffic to justify a proposal to add two more lanes to State Highway 23, at a cost of $146 million. Critics questioned these numbers, noting that car travel in Wisconsin appeared to have peaked way back in 2004 and that young people seemed to be driving less than previous generations.

In May, a district court agreed with the critics ... sort of. Judge Lynn Adelman ruled that Wisconsin's traffic projections were "inadequately described" — they were essentially inscrutable to outsiders. Adelman didn't have an opinion on whether car travel in Wisconsin had actually peaked. But he agreed that if car travel was peaking, then the case for expanding the highway became much weaker. At the very least, the DOT needed to show its work more clearly. (See Eric Jaffe at CityLab for more on this case.)

To some transportation reformers, even a little transparency would be a big deal. "A big part of the reason these flawed forecasts have continued to be made — and not corrected — is that the forecasting process is opaque to outsiders," writes Joe Cortright at City Observatory. "The federal district court’s ruling [in Wisconsin] should make it much more difficult for highway builders to continue justifying projects based on this kind of 'black box' modeling."

Illinois canceled a highway project after critics questioned inflated usage predictions

Hold that pavement. (Shutterstock)

Exhibit B: Transportation planners in Illinois had long been pushing the Illiana Expressway, a proposed $1.5 billion, 47-mile toll road to Indiana that would purportedly help relieve truck traffic off of I-80.

But that project took a hit when Chicago's Metropolitan Agency for Planning pointed out that only 20,000 vehicles were likely to use the expressway each day — about one-third of planned capacity and well below initial projections. That meant tolls wouldn't be able to cover the expressway's costs, and because of the way the project was financed, taxpayers would be on the hook for the shortfall, potentially diverting public money away from useful projects elsewhere.

This past May, environmental groups filed suit against the expressway, citing the harm the project could do to prairies as well as those inflated population and traffic forecasts. They didn't have to wait long. On June 2, Illinois Gov. Bruce Rauner announced that he was removing the expressway from his list of approved projects, and it's frozen for now.

Maryland and Washington have now stopped predicting that driving will keep soaring

Some states, meanwhile, are taking a more proactive approach. In the past year, both Maryland and Washington have revised their projections for future traffic growth significantly downward — they're no longer predicting that vehicle travel will keep rising indefinitely.

Here, via Sightline's Clark Williams-Derry, is how Washington's projections changed:

(Washington Office of Financial Management via Sightline)

It's a huge shift, and it means Washington's planners may look more skeptically at proposals for new highway projects in the future. There still very may well be a case for widening or building new roads in the future, but they won't be based on outdated metrics.

Meanwhile, over at Streetsblog, Phineas Baxandall points out that the federal Department of Transportation is beginning to revise its forecasts, too, coming around to the idea that vehicle miles traveled will grow much more slowly in the years to come than it did for most of the 20th century.

Granted, not everyone is rethinking things. Plenty of state and regional transportation agencies around the country are holding firm to the belief that driving will rise indefinitely. Last October, Streetsblog's Angie Schmitt pointed out that Dallas's local transportation agency was using strikingly high traffic projections — projections that exceeded even the wildest dreams of Texas state planners — in order to justify a 28-mile toll road between Garland and Greenville. At the same time, this project is getting scrutiny from local outlets like the Dallas Morning News, a sign that agencies are no longer getting a free pass on projects like these.

There are, potentially, better places to spend highway money

Back in 2013, the Frontier Group put out a report urging transportation planners to adapt to the end of the driving boom. Now that more states are starting to heed that advice, it's worth revisiting some of its recommendations.

The most straightforward idea was that states should plan for uncertainty. No one can predict the future perfectly, and it's entirely possible that the current plateau in vehicle miles traveled won't last forever. And there are undoubtedly still going to be cases where road expansions make sense. But transportation planners can "prioritize those projects that are most likely to deliver benefits under a range of future circumstances," rather than betting solely on soaring future demand.

Another key point was that states (and the federal government) should probably spend less money on building new roads and more money on maintaining existing roads. Economists have argued that more states should probably be doing this anyway — this Brookings Institution paper by Matthew Kahn and David Levinson makes the most comprehensive case. But the argument gets even stronger if vehicle travel is going to grow more slowly or plateau in the future.

What's clear is that it doesn't make much sense for states to keep planning around the idea that driving will grow at 20th century rates forever. Those predictions have been utterly wrong for nearly a decade now — and sticking with them could mean wasting billions on unnecessary roads and highways.

Further reading

-- We arguably spend too much money building new roads and not enough repairing old ones. Here's the case for fixing that.

-- My colleague Joseph Stromberg has a great piece exploring why driving declined among young people since 2007. It's extremely difficult to tease out how much was a temporary dip due to the recession vs. how much was a permanent drop due to attitudinal shifts, which is why it's worth being cautious about future projections.

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