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Expanded I-35 could induce demand and make pollution worse, transportation experts say

Aerial view of I-35 looking north from Woodland Avenue.
Nathan Bernier
TxDOT's plans for I-35 through downtown Austin could paradoxically make traffic worse and increase pollution by luring more drivers to the highway, a phenomenon studied by experts as "induced demand."

The Texas Department of Transportation's $4.9 billion proposal to increase the capacity of Interstate 35 through Central Austin could help reduce pollution if travel times were cut in half as predicted by TxDOT's own analysis of the project.

The problem, traffic experts say, is that such benefits — if they materialize — are usually short-lived.

The two leading proposals advanced by TxDOT would remove the upper decks, lower the freeway so you could see across it, add two non-tolled high-occupancy vehicle lanes in each direction and broaden the highway's footprint to as many as 20 lanes in some areas, including frontage roads. The plan includes widened bridges and enhanced infrastructure for pedestrians and public transit.

The state forecasts that when the new I-35 opens, traveling the main lanes from Ben White Boulevard to U.S. 290 East during afternoon rush hour would take either 39% or 50% less time than it does now, depending on which of two build options is advanced.

"The worst thing that can happen for air quality is what happens today, traffic sitting and idling and not moving," TxDOT Austin District Engineer Tucker Ferguson told Travis County commissioners this week. "Looking at the projections of population growth and what traffic may be added to the system, if we do nothing and not address that capacity expansion what we need, that's only going to get worse."

Emissions experts agree that vehicles moving more quickly generally produce fewer emissions per mile. That affects everything from the amount of human-made carbon dioxide warming the planet to volumes of inhalable particulate matter than can aggravate asthma and lead to premature death in people with heart or lung disease.

"Lower speeds have higher emissions," said Madhusudhan Venugopal, an associate research scientist with the Texas A&M Transportation Institute. "You're looking at the sweet spot is 35 to 45 miles per hour or so ... up to 65 or 70 and then [emission levels] slowly start curving up."

Induced Demand

The concern among transportation engineers is that increasing the highway's capacity would incentivize more people to use the interstate in the short-term and encourage sprawling development in the long-term. The more drivers are lured to the roadway, the slower traffic moves and the worse pollution gets.

"Any time you make something easier or cheaper, you see more demand for it," said Kara Kockelman, a professor of transportation engineering at UT Austin. "Something so in demand where there is serious congestion, you will definitely see more traffic because you've made that route easier and people will take advantage of it."

View of I-35 in icy and rainy conditions
Gabriel C. Pérez/KUT
Cars drive in icy and rainy conditions on I-35 during February's winter storm.

The T Word

In 2017, TxDOT announced a plan to manage some of that demand by adding two toll lanes in each direction. Pricing would go up or down depending on how busy the lanes got, similar to the MoPac Express Lanes. Such a pricing structure is backed by some experts as an effective way to manage demand for a popular public resource in a fast-growing region.

"It's not just roads. Anything that is underpriced — golf courses, bowling alleys, food at grocery stores — you're going to get overconsumption, and you're going to get congestion," said Robert Cervero, a professor emeritus of city planning at the University of California-Berkley. "It's just how supply-demand curves work."

Toll sign on the MoPac Express Lanes
Gabriel C. Pérez/KUT
A sign shows the toll rate on a MoPac Express Lane. The tolls vary depending on how busy they are with the goal of keeping traffic moving.

But a public backlash against politically unpopular tolled highway projects led Gov. Greg Abbott to call for a moratorium on them in November 2017, just weeks after TxDOT's announcement that the I-35 project would include managed toll lanes.

The Texas Transportation Commission voted unanimously in December 2017 to remove all new tollway projects from a long-term construction plan, effectively killing the toll plan.

Under the new plan, the four additional lanes would be managed by limiting them to high-occupancy vehicles and public transit. The definition of high-occupancy, which in Texas starts at two people, could be increased depending on the circumstances.

"Let's say [HOV lanes] are more popular than what we originally expected. One of the things we can change is the number of people [required] in the vehicle in order to use it," the TxDOT engineer Ferguson said. "So it takes more people to pile into vans and cars to carpool."

A TxDOT illustration showing a cross-section of what I-35 would look like near Capital Plaza in North Austin under Build Alternative 2.
A TxDOT illustration shows a cross-section of what I-35 would look like near Capital Plaza in North Austin under Build Alternative 2.

Electric Cars Are Coming

Construction on I-35 Capital Express Central, the project's official name, is not expected to begin until 2025 and will take years to complete. By the time the roadway opens, far more electric vehicles are expected to be on the roads than the 15,000 plug-in electric vehicles currently registered in Travis and Williamson counties.

General Motors, for example, says it will stop making gas-powered vehicles by 2035. Even gas-powered cars and trucks have become less polluting. Carbon dioxide emissions from new vehicles are at or near their lowest on record, according to the U.S. Environmental Protection Agency.

Electric vehicle charging stations in the Seaholm District
Gabriel C. Pérez/KUT
Austin Energy has been installing DC fast-charging stations, like these on Electric Drive in the Seaholm District, as more people switch to plug-in electric vehicles.

"There'll be more vehicles on the road, but a lot of them will be non-gasoline, non-diesel vehicles," said Sandeep Kishan, an expert in transportation emission modeling at Eastern Research Group in Austin. "That also will perhaps lead to a change in the emissions footprint of the whole fleet."

More Than Just Cars And Trucks

Environmentalists are enthusiastic about evolving vehicle technologies but say it's not enough to reduce emissions as much as necessary to avert catastrophic global warming.

"When you're trying to lose weight, we can't just eat a box of Twinkies and think an occasional celery stick is going to help. Your best strategy is to focus on diet and exercise," Environment Texas executive director Luke Metzger said.

"We need to power our cars with electricity, but we also need to give people more alternatives to driving, including public transportation, more work-from-home options, safe walking and biking, rather than having people continue to be stuck in traffic, yelling at the cars in front of them," he said.

Metzger is also concerned about the carbon footprint of a $4.9 billion highway project and years of construction-induced gridlock that will leave vehicles idling in traffic.

Even environmental activists like Metzger believe I-35 needs to be improved. He's most enthusiastic about one of three community drawn proposals examined by TxDOT that would shrink the interstate into an urban boulevard, part of a growing movement in the U.S. to remove highways constructed in urban centers. That plan, known as Rethink35, has been dismissed by TxDOT as insufficient to meet traffic demand.

TxDOT argues it has incorporated some community feedback into its designs already by, for example, improving pedestrian bridge crossings, lowering speeds on the frontage road to 35 miles per hour in the urban core and ensuring I-35 designs can accommodate the voter-approved transit expansion known as Project Connect. TxDOT says the I-35 expansion will yield other benefits like safer traffic designs that reduce crashes.

TxDOT rendering illustrating a concept of how Capital Metro's Red Line could cross I-35 at 4th Street.
TxDOT produced this conceptual illustration to show how Capital Metro's Red Line might cross I-35 at Fourth Street.

But for those primarily concerned about lowering the amount of climate-warming gasses in the atmosphere or reducing toxic vehicle emissions that cause illness, those potential benefits won't outweigh the costs of a major highway widening project.

The deadline to submit a comment to TxDOT to be included in the public record ends Friday night at 11:59.

Animation showing traffic on I-35 at Woodland Avenue
Nathan Bernier/KUT
Early afternoon Friday traffic on I-35 at Woodland Avenue

Nathan Bernier is the transportation reporter at KUT. He covers the big projects that are reshaping how we get around Austin, like the I-35 overhaul, the airport's rapid growth and the multibillion-dollar transit expansion Project Connect. He also focuses on the daily changes that affect how we walk, bike and drive around the city. Got a tip? Email him at Follow him on X @KUTnathan.
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