Transportation is the largest source of greenhouse gas emissions in the United States and accounts for half of the total increase in U.S. emissions since 1990. So you would think that progressive cities and states, with leaders nominally committed to reducing emissions and fighting climate change, would be doing their best to reduce dependence on cars and trucks, which account for 80 percent of transportation emissions.

Not exactly.

In Portland, Oregon, city and state officials are pushing a plan to spend a half-billion dollars widening Interstate 5 as it passes north of the city’s downtown. The project entered its period of public comment earlier this month. In a nod to the ugly history of urban renewal, the widening would take a chunk out of the backyard of Harriet Tubman Middle School. But it would also, the Oregon Department of Transportation says, dramatically improve highway travel times in the year 2045. The agency even says the newer, bigger highway would be better for the environment.

Not everyone is buying the argument. Aaron Brown, an organizer with the Portland group No More Freeway Expansions, sees the I-5 dispute as a microcosm of the larger, national struggle to slow the highway-industrial complex. “We’ve got 11 years to stop climate change, and if we’re making investments in intergenerational infrastructure, how can half-a-billion dollars be spent on freeway widening?” he asked.

Brown says the Oregon DOT—like other state transportation agencies whose interest and budget is committed primarily to road projects—has put its fingers on the scale to make the wider highway look like a no-brainer.

For example, the state says the corridor has some of the highest vehicle crash rates in Oregon and that the expansion would make the road safer. But Kristin Eberhard at the Sightline Institute notes that both recent fatal accidents on that stretch involved collisions with men walking on the highway—not a problem that would be solved by smoother traffic flows.

Most brazenly, in an environmental assessment, the state claims a wider highway would actually result in lower emissions in the neighborhood by 2045, thanks to faster-moving cars. (Angie Schmitt calls it the “greenwashing of a Portland highway expansion.) But it doesn’t address the possibility that the expanded road—like so many highway expansions—might draw in more drivers, making more trips, and creating more emissions.

In fact, researchers from Portland State University conclude that reduced congestion leads to more driving, and that induced and suppressed demand “are critical considerations when assessing the emissions effects of capacity-based congestion mitigation strategies. Capacity expansions that reduce marginal emissions rates by increasing travel speeds are likely to increase total emissions in the long run through induced demand.” In other words, traffic is more like a gas than a liquid, expanding to fill whatever space is created for it. Bigger highways invite more trips.

“The estimates they’ve given us are entirely a black box,” says Joe Cortright, a Portland consultant at Impresa (and the founder of City Observatory, where he has written about the I-5 project).* “There are no average daily traffic numbers, the most fundamental number you’d use to describe the volume of traffic.” (It wouldn’t be the first time the Oregon DOT miscalculated the environmental benefits of building new roads.)

More fundamentally, the project—an enormous investment in expanding a highway in one of the country’s most environmentally conscious regions—demonstrates the difficulty of slowing the road-friendly bias that permeates state transportation departments. Not only do new roads encourage environmentally destructive transportation and land use; they’re also a bad deal that has led the country into a road maintenance crisis. Existing road miles outnumber new ones 99 to 1, but states spend more money making those incremental additions than taking care of the rest.

As Alex Baca wrote in Slate a few weeks ago, any attempt to address America’s carbon emissions has to reckon with emissions from transportation (and highway-dependent land use). Building bigger highways through cities has been our approach for nearly a century. Is it still?