Calling it a necessary "investment in Nashville's future," Mayor Megan Barry on Tuesday unveiled a monumental proposal for a $5.2 billion mass transit system, the most expensive and complicated project in Metro history.

Barry wants Nashvillians to go to the polls in May to approve via referendum raising four taxes to pay for the massive undertaking — a combination of 26 miles of light rail, expanded bus service and a massive tunnel below downtown that would serve as a central connecting point for the city’s new transit lines.

Leading the way as a revenue generator would be a one-half percent hike to the sales tax that would jump to 1 percent in 2023. She’s also proposed increases to the city’s hotel-motel tax, rental car tax and business and excise tax.

After months of behind-the-scenes work on the effort, Barry presented the legacy-defining — and potentially controversial — project to city leaders at Music City Center on Tuesday morning. She said more transportation options will improve Nashville’s quality of life and make Nashville a better place for generations.

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“This comprehensive transportation solution will connect more neighborhoods with each other and open the door even wider to the city’s job, education and entertainment centers,” Barry said. “We will make sure that no one is left behind.”

Dubbed “Let’s Move Nashville,” the entire project has a timeline 15 years, with improvements to existing Metro Transit Authority bus services on tap in 2019. But the first light rail line would not open until 2026.

Light rail key part of proposal

The city would build light rail with dedicated lanes on Gallatin Pike, Nolesnville Pike, Murfreesboro Pike, and Charlotte Avenue. The plan also calls for a Northwest Corridor line that would run on existing railroad lines and connect from Charlotte Avenue through North Nashville past Buchanan Street and toward Bordeaux. There are also plans for a direct light rail connect to the Nashville International Airport.

A 1.8-mile downtown, three stop tunnel — first reported by The Tennessean last week — would stretch underground from Music City Central, the city’s bus hub, north-south under Fifth Avenue South to Lafayette Street, where an additional hub would be built.

There would be an access station for passengers at Lower Broadway. The tunnel, which by itself would cost nearly $1 billion, would be used for both light rail and new electric buses.

Existing bus service improvements would include the complete move to new electric buses, additional crosstown routes, 15-minute peak service on busy routes, more frequent bus service that would run 20 hours out of the day, and a reduction on some trips by as much as 80 percent.

The city has proposed new rapid bus lines for Dickerson Pike, Hillsboro Road, West End Avenue and a route that goes to Bordeaux. This type of transit would use limited curbside bus lanes instead of dedicated lanes and has more frequent service, and enhanced traffic signals and improvements to bus stops such as level-platform boarding.

Around two-dozen neighborhood transit centers would be built on locations along both the light rail and rapid bus lines. They also include the new downtown light rail hubs at Music City Central and Lafayette Street. Phased in over a five-year period, these hubs would offer access to the transit system feature park-and-ride areas, bike facilities and sidewalks.

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Much of Barry’s plan is detailed in nMotion, a 25-year transit plan adopted by the Metro Transit Authority and Regional Transportation Authority last year.

But while nMotion outlined a $5.97 billion for the entire Middle Tennessee region, Barry’s plan is nearly as much, $5.2 billion, just for Davidson County. Project leaders say the cost grew because of more detailed engineering work and the downtown tunnel, which was not part of the nMotion plan.

Early on in design work, project leaders deemed a tunnel under downtown Nashville necessary because of narrow streets and complicated right-of-way acquisition that would have been required to build rail above ground in the central business district.

Although some have observers had seen underground transit as a non-starter because of Nashville’s hard limestone geology, the mayor’s office says it can be done with new drilling technology. Seattle and Pittsburgh are among the cities that have done similar drilling to what Barry has in mind.

Tax increases to cover costs

The mayor has for weeks telegraphed what would be coming in regards to taxes to pay for the project.

The Tennessean last month reported that a sales tax would be part of the proposal. Barry later said to expect four tax revenue streams in all, and the mayor’s campaign committee conducted a telephone poll last week that singled out the taxes that would be part of it.

The sales tax hike would increase Nashville’s rate from 9.25 percent to 9.75 percent beginning July 2018 and 10.25 in 2023. The mayor’s office is highlighting data suggesting 47 percent of sales tax collections are from out-of-county visitors. The new sales tax surcharge passed by the state in the spring allowed Nashville to go above the state’s current cap of 9.75.

The mayor has proposed a quarter surcharge on the hotel-motel tax beginning next year. It would jump to three-eighths in 2023, pushing it to a rate to 6.375 percent overall – one of the highest in the nation.

A 20 percent surcharge would be added to the local car rental tax that would move it from 1 percent to 1.2 percent

A 20 percent hike has been proposed for the city’s business and excise tax. For a business that currently pays $1,000 in taxes, the increase would mean an additional $200.

Metro Council must act before voters

Barry’s administration plans to submit legislation to the council in December for approval to add the referendum tax proposal to the May 1 local primary ballot. It would need to be approved more than 90 days out of the election date.

The tax proposals are expected to generate around $110 million in annual revenue for transit beginning in 2019, ramping up to around $200 million by 2023.

Under the proposal, local taxes would sunset after 50 years. The state’s approval in the spring of Gov. Bill Haslam’s IMPROVE Act gave the new transit option to Nashville and other municipalities. In addition to the local taxes, Barry said the city plans to identify federal grants and funding to help pay for the project.

Eliminating fares for low-income residents

As part of the transit proposal, Barry has proposed eliminating bus and transit fares for Nashville residents who live at or below the federal poverty line.

She also said she intends to create a new affordability and transit task force to address needs of working families around transit. The mayor’s office said that means “shape policies” as it relates to affordable housing and supporting small businesses.

The mayor’s office has organized a series of six meetings over the next two months to discuss the transit plan.

Reach Joey Garrison at 615-259-8236, jgarrison@tennessean.com and on Twitter @joeygarrison.