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Traffic proceeds along Interstate 35 on the North Side during rush hour on June 29, 2018. Air pollution from car exhaust that contains nitrogen oxides is one factor in ground-level ozone, a primary component of smog, in San Antonio.

Traffic proceeds along I35 on the North Side during rush hour on June 29. A reader scoffs at city council’s plan to have all electric vehicles by 2050.

By 2040, Interstate 35 will run through a San Antonio-Austin metroplex with a population rivaling the Dallas-Fort Worth area’s current size.

And if nothing major changes by then, near-constant gridlock driven by the population boom will bog down what already is one of Texas’ most congested highway corridors.

To keep I-35 drivable, the San Antonio- and Austin-area Metropolitan Planning Organizations — local transportation planning entities for urban areas of more than 50,000 people — are coordinating on a nascent plan to add a pair of lanes in each direction to some 120 miles of the highway, from Bexar to Williamson County.

Officials would designate one of those lanes going each way as a “managed” lane, allowing them to adapt its use around evolving technology, including the proliferation of autonomous transit. Capital Area MPO Chairman Will Conley said he considers the potential lanes a “petri dish of all kinds of different options.”

Though ultimately Conley and Alamo MPO Chairman Kevin Wolff intend for the managed lanes to accommodate driverless vehicles, from cars to shuttle buses, the designation also would allow the lane to operate as a high-occupancy vehicle (HOV) lane for transit vehicles and carpools.

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The idea, Wolff and Conley agree, is that nobody knows what type of transit will be most popular, affordable or efficient in a decade, so the I-35 expansion lanes should be flexible enough to accommodate as many options as possible.

The Texas Department of Transportation already has agreed to pitch in $720 million, through its annually updated Unified Transportation Program, on a lane expansion from Loop 410 north to Comal County’s FM 3009. The Alamo MPO voted June 25 to support TxDOT’s recommendation for the project, which would add three non-toll express lanes in each direction.

The lanes, which would be mostly elevated, include one in each direction designated as an “HOV/special purpose” lane that eventually could provide for autonomous vehicles. Officials could award a contract for the project by 2020, at the earliest.

Wolff and Conley’s slowly unfolding plan would take years, if not decades — and that’s if the MPOs can find the $8 billion to $9 billion that Wolff, a Republican Bexar County commissioner, anticipates the lanes would cost.

Wolff estimated he and Conley would have to identify funding streams within the next three to five years to ensure segments of the expansion plan are ready for public use within the next 10 years.

“I wish I could say otherwise, but everything has to go perfect for that corridor to work,” Wolff said. “The reality is, from an infrastructure building standpoint, we are always behind the curve. Growth beats us every single time.”

Meanwhile, other funding could come from the federal government.

Wolff already has pitched the I-35 expansion plan to the U.S. Department of Transportation in hopes of securing funding from President Donald Trump’s estimated $200 billion infrastructure plan — about 10 percent of which is set aside for “innovative” transportation ideas, Wolff told the San Antonio Express-News, adding that DOT officials acted “receptive” to his proposal.

Otherwise, Wolff said he hopes to meet with Gov. Greg Abbott after the November elections — if Abbott wins his re-election challenge — to discuss the governor’s thoughts on paying for I-35 expansions without toll money.

Wolff has criticized Abbott and Lt. Gov. Dan Patrick over their opposition to toll roads, which in December prompted the state transportation commission to abandon planned toll lanes on parts of I-35. But Wolff isn’t ready to give up on tolls yet.

“While current state-level administration has put a hold on any toll-funded projects, that doesn’t mean tolls are gone forever,” Wolff said. “If you ask me today where to get the funding, I’m going to tell you tolls, because that’s the only real source of funding I can attach to right now.”

Conley, a Republican who’s running for Hays County judge, said he wants the plan to give commuters “the flexibility to make their own choices,” without necessarily forcing them onto toll roads from non-toll roads. And he said, “There’s not one project done today that’s not a smorgasbord of many different funding streams.”

Meanwhile, TxDOT and both MPOs are conducting a $1.5 million study, scheduled for fall completion, to develop a broader transportation strategy for Bexar, Travis and 10 surrounding counties.

The results of that plan are critical for how the I-35 expansion unfolds because new lanes would need to account for what Wolff called “ancillary stuff” — on- and off-ramps, and connections to alternate or nearby routes such as Texas 130 and U.S. 281.

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Those highways present a rare alternative to I-35; a recurring theme at a November meeting in New Braunfels between the MPOs’ transportation policy boards was the lack of transit options along the corridor.

Officials have grappled with this barrier for years, with some working toward a high-speed rail line. But the Lone Star Rail District, after years of stunted progress, all but died when Union Pacific in 2016 said it would not allow commuter rail on its freight line along I-35.

Even I-35 itself has limited wiggle room. The four new lanes likely will be elevated along most of the 120 miles because doing so is less expensive than right-of-way acquisition, though some stretches, like the area between New Braunfels and San Marcos, might be expanded outward, Wolff said.

Constructing elevated lanes would further disrupt traffic along I-35, though, with so much of the expansion unaccounted for, it’s impossible to forecast the scale of delay, or how long construction would take.

But construction would take awhile. An ongoing 8-mile expansion of U.S. 281 that began construction last year, for example, is expected to be finished in early 2021, with a separate phase set to begin in 2019 and possibly finish in 2022.

Nonetheless, technological advances have allowed workers to accelerate construction and otherwise limit inevitable disturbance, said John Habermann, a research engineer at the Texas A&M Transportation Institute.

One strategy, which has been increasingly used in projects around San Antonio, involves workers building highway sections with precast concrete off-site, before lifting the structures into place with cranes.

“That way, you don’t have to do all the form work in the air,” Habermann said.

Alerting drivers of construction delays has become easier, too, through social media and other channels, Habermann said.

From Wolff’s perspective, the plan for managed lanes effectively merges rail and road into a “hybrid” form of transportation, at a fraction of the cost of building steel tracks.

“When you look at what autonomous technology can really do, you can see the volume far exceeds nonautonomous, and gets close to what you can do with traditional rail,” Wolff said.

University of South Carolina law Professor Bryant Walker Smith, who studies autonomous vehicles, said lanes dedicated to autonomous traffic could accommodate “high-speed platoons of closely spaced vehicles” more easily than ordinary travel lanes, which likely will see autonomous traffic before autonomous-specific lanes open.

But despite the benefits of these lanes, “comparisons with rail are speculative,” he wrote in an email. “The technologies, applications, and business cases of automated driving are still uncertain, and indeed the same might be said for rail. There may also be important differences between freight and passenger uses. I'd be wary of abandoning rail projects on the expectation that automated vehicles will be a suitable substitute.”

Others are skeptical that simply adding more lanes can fix I-35 congestion. A June report from the U.S. Public Interest Research Group’s Education Fund and Frontier Group called a separate I-35 expansion plan through Austin a “highway boondoggle.”

However, that plan does not emphasize driverless vehicle lanes as Wolff and Conley intend with their plan. The report also criticized the Austin plan for ignoring “the growing need for transit and complete streets to create more compact and connected neighborhoods.” Much of the Wolff-Conley four-lane expansion plan would pass through less populated areas.

Ultimately, Wolff doesn’t want just to prevent I-35 from getting out of control. He wants to reduce the commute between San Antonio and Austin to less than an hour.

“If we can connect these two cities, we go from two competing areas of roughly 5 million people to one regional area of over 7 million people tomorrow,” Wolff said. “Then can you imagine going after the Amazons of the world?”

Jasper Scherer is a San Antonio Express-News staff writer. Read more of his stories here. | jscherer@express-news.net | Twitter: @jaspscherer

Jasper covers Texas politics for the Houston Chronicle's Austin Bureau. He previously reported on City Hall and local politics at the Chronicle and covered local government for the San Antonio Express-News. He graduated from Northwestern University in 2017 with degrees in journalism and political science, and has interned for the Tampa Bay Times, Washington Post and Fortune magazine.