Do bigger highways actually help reduce traffic?

From Massachusetts to California, transportation departments are pursuing controversial plans to widen highways, expansions that are sure to compel more people to drive, thus increasing greenhouse gas emissions. Yet state and federal officials are, absurdly, justifying such projects by claiming that they can help fight climate change.  

Consider a report issued last fall, in which the Texas Department of Transportation (TxDOT) outlined its strategy to reduce pollution attributable to its road network, which a 2018 department report found generated 0.48 percent of all global — not national — CO2 emissions. Along with improving public transit, installing energy-efficient streetlights, and building electric vehicle charging stations, TxDOT suggests expanding highways.

According to TxDOT’s report, projects like adding turnaround lanes on frontage roads will reduce emissions because they “[reduce] vehicle idling due to delay.” State DOTs from Utah to New York have likewise claimed that adding lanes to congested highways will lower emissions because fewer cars will be stuck in traffic.

Framing highway widening as a cure for climate change has allowed state DOTs to justify spending billions of dollars in their ongoing war on gridlock. Businesses and residents alike complain about traffic, and widening the road is an easy way to placate them because it feels like progress. But decades of research — along with common sense — show that congestion will inevitably return. New roadway lanes invite more cars, which generate more emissions, trapping us in a cycle of ever-increasing driving that only makes it harder to slow the increase in global temperatures.

How could they possibly be saying bigger highways are good for the climate?

It’s worth pausing to consider how state DOTs justify conclusions that seem so far off-base. When considering potential highway projects, staff use computer models to forecast their impact on future traffic. These models project that driving will grow at a rate reflecting past trends, often with a bump for population expansion. Any gas-powered car will create emissions when driven, but one stuck in gridlock will produce more since its journey takes longer.  

State DOTs’ rigid assumptions about driving growth lead them to predict that traffic will eventually overwhelm the existing highway network. “Their thinking is, ‘if we don’t do anything, these cars are going to be sitting on this highway and not moving,’” Wes Marshall, a licensed traffic engineer, urban planning professor at the University of Colorado-Denver, and author of the new book Killed by a Traffic Engineer, told me. “If that’s the baseline condition, any [expansion] is going to be better.”

It’s a nice, tidy story — but it’s totally wrong. 

These projections have a fatal blind spot: They fail to consider how humans respond to changing conditions like new vehicle lanes. When people see cars traveling freely over a recently expanded highway, they will recalibrate their travel decisions. Some will choose to drive at rush hour when they would have otherwise driven at a non-peak time, taken public transit, or perhaps not traveled at all. When a roadway is widened, Marshall said, “You might have less congestion at first, but it quickly goes away.”

Such behavioral adjustments will continue until traffic is as thick as it was before, when the roadway was narrower. The only difference is now there will be more cars stuck in traffic, emitting even more pollution. 

This phenomenon is known as induced demand. In his book Fighting Traffic, historian Peter Norton notes that as early as the 1920s, a New York City engineer warned that new roadways “would be filled immediately by traffic which is now repressed because of congestion.” In the 1960s, the economist Anthony Downs wrote a seminal economics paper that codified the concept, which has been called the Iron Law of Congestion. As one researcher put it, “If you build it, they will drive.”

Induced demand is the bane of highway expansion projects. In Houston, average rush-hour journey times on the Katy Freeway lengthened by 15 to 20 minutes three years after TxDOT spent $2.8 billion widening it to as many as 26 lanes (including frontage lanes) in 2011. In England, researchers examining the expansion of the M1 motorway north of London found that “traffic moved more slowly than before the scheme opened.” The blunt conclusion of a 2011 study in the American Economic Review: Adding road lanes “is unlikely to relieve congestion.” 

If highway expansions don’t relieve gridlock, they cannot reduce emissions. To the contrary, they worsen them. As a 2012 study put it: “In the long run, capacity-based congestion improvements … can reasonably be expected to increase emissions of CO2e, CO, and NOx through increased vehicle travel volume.”

The total environmental toll of roadway expansions looks even worse when considering the second-order effects. Wider highways convince more people to drive, which may increase car purchases — and once people own a car, they tend to use it. Expanded roadways could compel some to relocate to bigger homes that sprawl further from the urban core, elongating commutes. The billions of dollars that state DOTs are allocating toward a Sisyphean war on congestion could instead be spent on projects that can credibly reduce driving, such as mass transit and dense development. 

“By adding more lanes to a highway, you’re inducing more car-oriented land uses,” Marshall said. “Zooming out, you’re creating a much more auto-oriented environment, not just for that one roadway, but for the whole area.”

The faulty logic is hard to dislodge

Nevertheless, the idea that wider highways are good for the planet remains widespread within state DOTs, including in blue states where officials cultivate an image of environmental stewardship. Oregon’s DOT used it to justify its proposal to widen I-5 in Portland in 2019, and California’s transportation department continues to argue that widening I-80 between Sacramento and Davis would reduce emissions, an assertion that environmental groups are challenging in California state court.

To be fair to state DOTs, this misconception is enshrined in federal policy. In the early 1990s, Congress created the Congestion Mitigation and Air Quality program, whose very name implies a linkage that does not necessarily exist. Its funding, now totaling $2.6 billion per year, has gone toward climate-friendly investments in bikeshare in the District of Columbia and the MBTA Green Line in Boston — but also toward highway widening projects such as adding lanes to I-10 in Los Angeles County. (Over email, an FHWA spokesperson did not answer directly when asked whether the agency believes that roadway expansions reduce total emissions, responding that the agency “provide[s] an array of tools and programs to help mitigate congestion impacts.”)

To this day, federal policymakers struggle to acknowledge the linkages between highway construction and pollution. Speaking in May at an event celebrating new lanes being added to I-25 north of Denver, FHWA administrator Shailen Bhatt said, “By eliminating the bottleneck between Mead and Berthoud … we’re advancing safety, trip reliability, freight efficiency, and reducing emissions.” 

Clear thinking on such matters is difficult due to powerful political pressures behind highway construction, which generates thousands of jobs and billions of dollars in business for contracting firms. In May, the head of a California alliance of labor and business groups declared it a “false equivalency” to claim “we cannot meet our climate change goals and not continue to invest in our roads, bridges and highways.” 

In reality, striving to reduce emissions while expanding roadways is like trying to become healthier while continuing to gorge on junk food.

The good news is that a small but growing number of state legislatures recognize the trade-off between environmental progress and roadway growth. Colorado and Minnesota, for instance, recently passed bills requiring state DOTs to minimize the climate impact of their investments. New projects that enable drivers to take shorter trips — or better yet, travel by riding transit or a bike instead of driving a car — ought to be able to easily pass muster, but highway widenings should not. In Colorado, several planned expansions have already been canceled because of the new rules. Maryland is considering similar legislation demanding “methods for evaluating induced demand in assessments that measure greenhouse gas emissions.” 

Marshall told me that he doesn’t know whether transportation officials claiming that roadway projects will curtail emissions are simply parroting what their faulty models tell them or whether they are knowingly spreading misinformation in order to keep building the projects that business and labor groups demand. Flawed though their models are, state DOTs have a political incentive to keep using them.

Regardless, the facts are clear: Rather than mitigating climate change, highway expansions exacerbate it. “There’s enough research out there showing again and again that it doesn’t work,” Marshall said. “You would think they would know better.”