Correction: An earlier version of this story misidentified a transportation agenda released by Mayor Megan Barry's office in May. It is called Moving the Music City.

As Mayor Megan Barry touts the urgency of addressing Middle Tennessee’s congestion with a $5.2 billion transit plan, a big question persists that could undermine her campaign: What about autonomous vehicles?

Self-driving cars and buses are being developed by the world's top tech and auto companies.

Meanwhile, the mayor’s proposal, involving light rail, rapid bus and a downtown tunnel, comes with a hefty price tag and a 14-year construction time frame.

If autonomous vehicles make significant strides in the coming years, transit needs could shift dramatically in Nashville and in cities around the world, complicating the push for the mayor’s "Let’s Move Nashville" plan.

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"Well within the time frame of these investments, you are going to have a different world out there," said Steve Polzin, director of mobility policy research at the National Center for Transit Research in Tampa, Fla. "As long as it's not more than 40 years away, the reality is, yes, there will be lots of automated services that will compete with it, well within its useful life.”

With increased congestion fueled by rapid population growth, government and business leaders across the region have been calling for meaningful action to preserve the area’s quality of life. A voter referendum on the taxes needed to fund the mayor's transportation plan is expected in May.

But Metro Councilman Robert Swope questions the need for expensive light rail and says the focus should instead be on self-driving solutions. While autonomous vehicles are not yet available for general ridership, he said the transit plan also would take several years to finish.

“In 15 years, no one will own a car anymore,” Swope said. "I can show you places around this world I have been to where Level 5 autonomous vehicles are in operation today and our state has already granted the legislation to make that a reality in Tennessee. Why are we not embracing this?"

Swope envisions fleets of autonomous vehicles operated through public-private partnerships. Metro could charge fleet operators pennies per mile and generate revenue while subsidizing low-income riders, as it subsidizes the city’s bus service today with $49 million.

The mayor’s office foresees autonomous vehicle fleets as part of the proposed transit system, providing cost-effective, last-mile and first-mile transportation to and from bus and light rail lines.

In May, Barry's office released a three-year transit road map called Moving the Music City, concluding autonomous vehicles should be "shared, electric and carefully integrated" to complement mass transit.

The city also is developing a self-driving vehicle model focused on improving transportation for low-income college students, Barry spokesman Sean Braisted said.

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Self-driving cars “will definitely be one part of our mobility future,” Braisted said. “But high-capacity transit will still be critical to moving large numbers of people over long distances, particularly during peak hours. There will always be constraints on how many cars, self-driving or standard, can fit into the region’s most congested areas. It all boils down to space, which dense and growing cities like ours inherently lack.”

Swope is in favor of a last-mile, first-mile self-driving vehicle solution, but says that is just the beginning of implementing automation.

While Metro buses would be converted to electric under the new plan, Braisted said autonomous technology that improves safety and efficiency could be implemented as well. Self-driving vehicle pilots could include shuttles, with backup drivers available to ensure safety and security.

The Nashville Area Chamber of Commerce's citizen-led initiative on transit, called Moving Forward, has pushed for an autonomous vehicle pilot program. It also has asked for a clearer vision from Metro on implementing self-driving vehicles in future planning.

"Moving Forward is convinced that AV technology can be a supplement to mass transit, but transit investment is needed to move the sheer volume of current and future Middle Tennesseans around the region," Moving Forward leaders wrote in a recent Moving Forward report on autonomous vehicles.

Transportation shifts underway with Lyft, Uber

Already, new technology has shifted demand for public transit, according to a recent study from the University of California-Davis. Ride-hailing services, including Uber and Lyft, have attracted Americans away from transit in major cities, leading to a 6 percent decline in bus use and a 3-percent decline in light rail use. Conversely, ride-hailing has boosted commuter rail usage by 3 percent.

About half of all ride-hailing trips would not have been made at all or would have been made by walking, biking or transit, and the service likely contributes to growth in vehicle miles traveled, according to the study.

Like public transit, an autonomous vehicle gives passengers the option of being productive as they travel. It also has potential to offer greater convenience and means a passenger is not captive to public transit schedules, Polzin said.

As for congestion, Polzin said it is up for debate as to whether autonomous vehicles can have a meaningful impact. The answer depends on the variables related to self-driving vehicles, he said.

Congestion could decrease if passengers in self-driving cars carpool, if vehicles become more compact and if cars drive much more closely together. Lanes could become smaller, freeing up more road space, and a main source of congestion — car wrecks — could be significantly reduced if the technology lives up to safety expectations, he said.

“A lot remains to be seen,” Polzin said. “There is uncertainty and that is a real dilemma that planners face.”

And the time frame for autonomous vehicle development and adoption is also unclear. Very smart people have very different answers, with estimates ranging across the next few decades, he said.

"We don’t want to say, ‘the world’s uncertain, so we shouldn’t do anything’ because the growth and demand and congestion could creep up on us," Polzin said. "On the other hand, there are some very legitimate scenarios where fixed-price, capital intensive investments might be facing different competition in terms of people’s choices down the road — 10, 20, 30 years, depending on which scenario of automated vehicles you believe.”

A 2015 study by the Organization for Economic Cooperation and Development determined that autonomous vehicles could remove 80 to 90 percent of cars in a mid-sized European city. If a high-capacity public transportation system is not in place and autonomous vehicles carry a single passenger, car travel volume would nearly double.

Not a binary choice

Mark Cleveland, local business leader and founder of carpooling app Hytch, has been a big proponent of advancing self-driving vehicles in Nashville. There is likely a place for a light rail option and autonomous vehicles in the city's transit solution, he said.

“It is not as simple as just one or the other. It is likely to be both," Cleveland said. “Autonomous vehicles will replace your car. They won't replace the rail systems."

Cleveland is also confident that drivers will be more willing to share cars as it becomes easier to do so. He’s betting his Hytch business model on that optimism.

"Autonomous vehicles are going to arrive and change the way people view mobility entirely," he said. "Both of those assets will benefit a growing Nashville.”

Reach Jamie McGee at 615-259-8071 and on Twitter @JamieMcGee_.