How much could different taxes generate for transit in Nashville region? There's now a calculator.

Nashville Mayor Megan Barry announced months ago plans to push for a public referendum next year that would let voters decide whether to create new tax surcharges to pay for transit.

But outside of saying she's eyeing the referendum for the May local primary election, she's been quiet about the details — including which taxes she wants to target and how much funding she hopes to generate.

While those questions remain unanswered, there's now an easy way to calculate the transit funding potential of four tax possibilities: increasing to sale tax, property tax, hotel/motel tax and wheel tax.

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On behalf of the Nashville Area Chamber of Commerce, William Fox, professor of business and economic and the University of Tennessee, has created new revenue forecast models, posted online, that projects funding capacity over time for each of these taxes.

"We've developed the capacity for you to develop and create your own revenue estimates," Fox said at a Wednesday meeting hosted by the chamber to roll out its Moving Forward transit group's latest report. "It is so easy to use. Anybody can run this model easily and quickly."

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Think of it as a calculator for Middle Tennessee transit funding, one that looks ahead 30 years from now.

There are different models for 10 counties including Davidson based on historic patterns and growth forecasts. It allows anyone to go online and see the potential of raising one of the four taxes by a certain percentage. The tool also allows one to see the impact of raising a combination of the four taxes.

For example, increasing the sales tax in Davidson County by 1 percent would generate $156 million in 2018, according to the calculator. Doing so by .0275 percent, would create $429.1 million in 2018 for transit.

If, say, Metro wanted to raise sales tax by 1 percent, the property tax rate by 53 cents and the wheel tax by $5, it would create $321.7 million for transit in 2018.

The combinations go on and on.

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The Metro Transit Authority has adopted a regional transit plan dubbed nMotion that outlines $6 billion worth of transit projects over 25 years.

Barry has proposed starting a light rail system on Gallatin Pike, but has not given a price tag.

Fox stopped short of recommending the use of one tax over the other to pay for transit, saying that question is for elected officials. But he did note one significant observation — it would take major increases in either the hotel/motel or wheel tax to produce significant funds for transit.

Sales and property taxes, on the other hand, have much greater funding capacity.

"The point is, you can very simply fill in the pathway and have a very good view of where taxes are likely to go and which tax source can generate the greatest potential," Fox said.

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Barry, during remarks Wednesday, expressed confidence that Nashville voters will approve a transit referendum measure in May. She also offered hope that there will be a shovel in the ground for light rail before 2020.

"I don't know about you, but there's something that happens to me in 2019," Barry said, a reference to her reelection date that year. "So 2020 sounds like a little bit too late."

To get approval for a May referendum on transit funding, Barry would have to receive at least two-thirds approval from Metro Council.

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