TAMPA — When it comes to finding the sweet spot for a new major league ballpark, the number of people living within a 30-minute drive may hold the answer.

It's why Major League Baseball has insisted that new ballparks go in densely populated urban areas. It also helps explain all those empty seats at Tropicana Field.

Just 615,000 people, less than one fifth of Tampa Bay's population, were estimated to live within a half-hour drive of the Rays current home in 2010 when the a group called the ABC Coalition compiled a study.

How much will that change if the Rays move to a shiny new ballpark in Ybor City?

The short answer: A lot.

If the Rays were to take the field today at —let's call it Cigar City Stadium — they would draw from a population of almost 920,000 people within a 10 mile radius.

Widen that to 15 miles and the numbers swell to 1.2 million people, according to an analysis by county planners in both Hillsborough and Pinellas conducted at the request of the Tampa Bay Times.

And unlike Tropicana Field, almost all of those potential fans could skip the trip across Tampa Bay and its congested bridges to catch home contests during the 162-game regular season.

How much that would raise actual attendance would depend on the income and age of the population, but putting a ballpark within easier reach of that many more people would help, said Patrick Rishe, director of the sports business program at Washington University in St. Louis.

"Getting from one part of Tampa Bay to another can be an issue," Rishe said. "There's no question that when you play so many games, proximity and convenience is vital."

Draw a circle 15 miles in diameter from the proposed site at the northeast corner of Adamo and Channelside drives and you almost reach the Pasco County border to the north and Apollo Beach to the south. It includes some of Hillsborough's most populous communities — Seminole Heights, Carrollwood, Tampa Palms and Brandon. It also includes parts of Pinellas County close to the Gandy and Howard Frankland bridges.

The circle, though, doesn't touch the many affluent subdivisions along Bruce B. Downs Boulevard.

How many people live within a 30-minute drive of the site is tougher to calculate because rush-hour traffic and seasonal weather vary so widely.

Sykes Enterprise CEO Chuck Sykes, co-founder of booster group Rays 2020, said at a recent community meeting he has calculated the population within a half hour drive at 1.6 million.

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That number may be a little optimistic measured against estimates developed with software used by utility companies and grocery store developers, among others, to measure rooftops and population around future projects. One program that uses average journey times along with 2010 Census data and other more recent population estimates puts the number at 1.4 million.

That's still more than double the comparable number for the Trop. These calculations also place communities as far away as Plant City within 30 minutes of an Ybor City ballpark because of their proximity to Interstate 4.

Rays' back office executives crunched similar numbers before they publicly backed the Ybor City site. The team, which has recorded the lowest average attendance in MLB for six straight years, says it needs bigger crowds to remain competitive.

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A consultant hired by the Rays projects that by 2025, when the team may be settling into new digs, some 1.6 million people will live within a half-hour drive of an Ybor City ballpark. That number would rise to 2 million by 2045, the consultant told the team, which shared some of its data with the Times.

"The Tampa Bay region has a unique geography, and the Ybor City site represents an opportunity to maximize our accessibility to the region's existing and growing residential and business community," Rays President Brian Auld said in a text message. "Its location allows us to leverage a come-early, stay-late culture in a dynamic, urban neighborhood."

The number of people clustered around a ballpark is critical when it comes to weekday games, which can account for close to half a team's 81-game home schedule but typically draw smaller crowds.

That's particularly true for the Rays, whose average weekday attendance has been as much as 5,000 lower than on weekends — a disparity that is among the worst in the league.

Applying a standard metric used by sports economists, an extra 600,000 people within reach of the ballpark could add an average of about 2,000 fans on weekdays, said John Vrooman, a Vanderbilt University sports economist.

That would boost the team's weekday attendance to about 75 percent of weekend attendance — a ratio comparable to the Pittsburgh Pirates, Kansas City Royals and Cincinnati Reds, he said.

But moving across Tampa Bay might mean saying goodbye to many long-time Rays fans, said Roger Noll, professor emeritus of economics at Stanford University, who researches sports business.

When the National Football Leagure's San Francisco 49ers left the city for Santa Clara in 2014, the composition of season-ticket holders shifted dramatically towards the area's South Bay. The Rays might experience something similar, he said.

And Noll warns that a new ballpark may only translate into a short-term boost in annual attendance.

While Tampa may be larger, it's population — a little poorer and younger than St. Petersburg —may be less attracted to baseball, he said.

"The initial honeymoon effect of a new stadium will increase attendance a lot — probably by a million or more," Noll said. "But the honeymoon will wear off in a few years, and the new location is probably worth an additional attendance of 200,000 to 300,000 a few years down the road."

Contact Christopher O'Donnell at codonnell@tampabay.com or (813) 226-3446. Follow @codonnell_Times.