Bigger freeways don’t cut traffic congestion, many studies show. What could Utah do with Interstate 15 instead?

Research shows that while highway expansion cuts travel times at first, traffic returns as people change their behaviors.

(Rick Egan | The Salt Lake Tribune) Homes border both sides of the I-15 freeway, in Woods Cross, on Thursday, Jan. 5, 2023. UDOT proposes to widen the freeway from Farmington to northern Salt Lake City.

This story is part of The Salt Lake Tribune’s ongoing commitment to identify solutions to Utah’s biggest challenges through the work of the Innovation Lab.

Utah Department of Transportation has models that show widening Interstate 15 through northern Salt Lake and southern Davis counties will cut projected drive times in half decades down the road.

But widening highways “clearly” doesn’t improve congestion, said Susan Handy, a professor of environmental science and policy at the University of California Davis.

Handy has authored and co-authored policy briefs summarizing research and studies on induced demand: The idea that highway expansion generates a proportional increase in vehicles using the roadway.

The explanation for why is pretty simple, Handy said. Widening the highway reduces the cost of drive time and people consume more when that cost goes down.

A wider highway succeeds at what it’s designed to do, she said, which is to carry more cars. It doesn’t succeed at the stated goal of reducing congestion in the long run, she said.

It’s not clear whether there is actually a way to solve congestion, Handy added, but it’s easier to live with when officials make it possible for people to avoid it by investing in high-quality, high-frequency transit service.

“It’s really about rethinking our communities to make them less dependent on all of this driving,” Handy said.

There’s increasing opposition to I-15 expansion plans from Salt Lake City’s west-side neighborhoods — from Rose Park, to Fairpark, to Glendale. And in a Salt Lake Tribune op-ed last week, Mayor Erin Mendenhall encouraged rigorous questioning and analysis about more highways. “History has shown that if you build more, more will come,” Mendenhall wrote.

The Utah Department of Transportation is looking at a holistic approach to transportation within the corridor between Farmington’s Shepard Lane and Salt Lake City’s 400 South, spokesperson John Gleason said.

“Widening I-15 is part of a comprehensive approach to meeting transportation demand through the year 2050 that includes added capacity to FrontRunner, additional bus service, local and regional roadway improvements and new facilities for those who walk and bike,” Gleason wrote in an email.

He added the current preferred option that widens the highway along the 17-mile stretch is one the state agency thinks balances travel improvement with community impact.

Why widen the highway?

Utah’s growing population and aging infrastructure necessitate looking at ways to improve mobility, Gleason said. That includes adding capacity to meet increasing demand, he said.

Based on a 2019 study, it takes about 18 minutes during morning peak times to drive from Farmington to Salt Lake City along I-15, compared to 19 minutes in the evening.

Using future growth travel models, UDOT projects it will take more than an hour to travel that same stretch of highway in 2050, Gleason said.

Presentations by the state estimate travel times of 55 minutes at morning peak times and 66 minutes in the evening.

Implementing UDOT’s preferred option of adding five general purpose lanes, an express lane and an auxiliary lane in certain areas in each direction results in a projected travel time of 30 minutes, Gleason said.

According to a presentation, it would take 28 minutes to get from Farmington to Salt Lake City during the morning rush hour and 30 minutes to head back north in the evening.

“While this is still an increase in travel time over today, we feel this is the best balanced travel improvement with impact to the surrounding community,” Gleason wrote.

A second option would be similar but would make the central express lanes reversible to serve morning and evening traffic demands. That would reduce morning travel time to 21 minutes and evening travel time to 22 minutes, according to the UDOT presentation.

Traffic returns as people change travel behaviors

But study after study has shown that while widening highways reduces travel times at first, traffic returns as people change their behaviors.

(Rick Egan | The Salt Lake Tribune) Victoria Petro represents Salt Lake City Council district 1, and is questioning a UDOT proposal to widen I-15 through her constituents' neighborhood in the northwest section of the city. Here, Petro speaks at her inauguration day on Jan. 3, 2022.

After hearing a highway is less busy, commuters might switch from transit to driving or change the route they take to work. Over time, people may move farther from work and developments may become more dispersed in response to the capacity increase.

A policy brief UC-Davis’ Handy authored in 2015 showed aid a 10% capacity expansion is likely to increased traffic by 3% to 6% in the short run and 6% to 10% in the long run.

In a more recent policy brief Handy co-authored, that was up to 3% to 8% in the short run and 8% to 10% in the long run.

People have observed and experienced this over decades, she said. Officials have continued to widen highways, she said, but congestion is still as bad as ever.

The forecasts by transportation departments don’t pan out, she said, and are flawed to begin with.

There’s a lot that can happen between now and 2050, including changes in technology, society and development patterns, Handy said.

“There are forecasts based on lot of assumptions. When you’re talking about that far in the future, there’s a lot that we just don’t know.”

UDOT accounted for changes in travel behavior — such as shifts to other modes of travel as they improve — in the coming decades, Gleason said.

Even so, Handy said, forecasts aren’t good at predicting the increase in driving that will result from a wider highway. They tend to over-predict the benefits while under-predicting the environmental impacts.

Congestion often self-corrects, she added, because there’s incentive for people to drive less as traffic gets worse. Forecasts also don’t take that into account, she said.

Why continue to widen highways?

A doctoral student at UC Davis is looking at why officials keep widening highways, Handy said.

It’s a lot of different reasons, but two stick out, she said: Highway expansion is the solution transportation officials have turned to since the 1960s, and many people have a financial stake in that approach.

(Francisco Kjolseth | The Salt Lake Tribune) Traffic moves along I-15 in the northern end of Salt Lake City on Thursday, September 3, 2020. Numerous studies show that building more highways or expanding freeway lanes like UDOT is proposing will not end traffic congestion.

UDOT and similar departments in other states used to be highway departments, Handy said.

“They were created with the mission of building highways, and have only expanded their mission in recent years. Highways are still the bulk of what they do.”

Drivers also demand wider highways because they don’t fully understand induced demand, she said, and policy makers agree to respond to voters’ wishes.

What could Utah do instead?

It’s not clear whether officials can solve congestion, Handy said.

But if they really want to reduce the number of vehicles on the roads, it will take pricing, she said.

That doesn’t mean just adding an express lane where people carpool or pay a toll.

“It would take tolling the entire roadway,” Handy said. “People are driving as much as they are because we are underpricing driving relative to its impact.”

Tolling roads isn’t politically feasible — especially in the west — but might eventually be the only thing that makes sense, she said.

People need to discuss whether congestion is a problem that needs a solution, she said. Nobody likes it, and there are negative consequences to traffic, Handy said, but it’s somewhat an inevitability of a vibrant city.

Congestion has only reduced during recessions and pandemics, she added, and it might just be time to learn to live with it.

Officials can help with that by making it possible for people to not have to deal with congestion, Handy said, whether that’s enabling people to live closer to work or improving public transit.

Transit needs to be high-quality, run frequently and give people more options for the “last mile” — the remainder of their trip to work or home. This last mile issue has plagued transit planning along the Wasatch Front for decades.

“It takes a very concerted effort, but the resources are there,” Handy said. “It’s a matter of how we choose to spend the resources.”

What are other states doing?

Utah is considering a holistic approach that includes looking at how to get people to their final destination safely and easily whether they’re driving, riding public transit, biking or hoofing it, Gleason said.

But some states have started to completely rethink road building and transportation spending, especially following federal guidelines that encourage a “fix it first” approach before adding new highway miles.

Colorado, for example, adopted a climate change regulation in December 2021 pushing transportation planners to redirect funding away from highway expansions and toward projects that cut vehicle pollution, such as buses and bike lanes.

That led government officials to approve updated long-term plans that remove a massive highway widening project along Interstate 25 in Denver and reinvest more than $100 million in transit and safety improvements.

Opposition from community and government officials led to the cancelation of a planned expansion for Interstate 710 through Los Angeles. Transportation officials axed the project even after spending $60 million on design and planning over two decades, according to The New York Times.

(Trent Nelson | The Salt Lake Tribune) Molly Segura with family in her Salt Lake City home on Saturday, Jan. 21, 2023. From left are Denece Martinez, Segura, Corine Giron, Becky Benavidez and Leah Benavidez. Known as "Grandma Molly" by her friends and family, Segura has lived in the Guadalupe neighborhood for decades and worries she'll be forced to move for the second time because of proposed I-15 expansion.

Meanwhile, Salt Lake City’s west-side residents are continuing to question the wisdom of I-15 expansion at the expense of air quality and possible razing of homes to accommodate the project. “It’d be really nice if this time our resilience wasn’t tested, and maybe our prosperity was invested in,” Salt Lake City Council member Victoria Petro told The Tribune in January. Petro represents neighborhoods in the project zone on the city’s northwest side.

It isn’t clear yet whether their efforts will result in any changes to the state’s plans, but Handy sides with west siders who are fighting the project.

“The city does not fall apart if the road does not get widened,” she said.

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