TAMPA — Reducing road congestion has been one of the sweeteners for the penny on the dollar sales tax pushed by All for Transportation, the group that put the plan on the Nov. 6 ballot.

But the $280 million per year tax won't fix congestion by itself, Hillsborough County Administrator Mike Merrill told a citizens advisory committee this week.

With Hillsborough growing by 40,000 people every year, the additional investment in transportation may only slow how quickly congestion grows, he said. For the tax to make a bigger difference, the county must take other politically difficult steps, such as discouraging developers from adding more car-dependent subdivisions.

Merrill said denser development like townhomes, apartments and condos are needed to dovetail with transit, which is part of the sales tax plan.

"The question is how rapidly can we dig our way out of a hole?" he said. "If we stop building acres and acres and acres of single-family homes that cost me more to serve than I get in revenue, that will help."

His comments were made just one week before Hillsborough voters decide whether to approve the transportation sales tax.

Merrill, who helped develop the Go Hillsborough plan that commissioners declined to put on the ballot in 2016, has avoided taking a position on the transportation initiative.

But he warned that congestion on roads will get even more dire if the county does nothing.

"Without a dedicated funding source, we'll never break gridlock," he said in an interview.

Merrill's comments are disputed by All for Transportation.

Its plan would provide about $124 million for bus and transit and about $149 million for roads, pedestrian safety and trails. It is based on priorities identified in the Hillsborough Metropolitan Planning Organization's long-range transportation plan.

The financial injection would help pay for hundreds of unfunded road projects like new turn lanes. Intelligent traffic signals that adjust to real-time traffic conditions could be added at about 640 intersections, including Kennedy Boulevard and Hillsborough and Nebraska avenues.

The plan will also makes roads safer with more crosswalks and sidewalks, and allow the county to round out a network of 400 miles of bike and walking trails.

With the sales tax, the MPO plan shows time savings of more than 20,000 hours per day by 2040, said All for Transportation Chairman Tyler Hudson.

"Without a doubt, the All for Transportation plan will reduce congestion. It's that simple," Tyler said.

Jean Duncan, Tampa's director of transportation and stormwater, said the traffic enhancements will benefit commuters.

"The public will feel the difference with the improvements we can make," she said. "This plan will be transformative in terms of congestion reduction, in our opinion."

Notaxfortracks.com, the group campaigning against the ballot initiative, said Merrill's comments show that All for Transportation's claims are bogus.

"They have not shown a single piece of evidence that supports their claim," said Jim Davison, a Tampa physician who has donated $2,000 to notaxfortracks.com.

Formed by some of the founding members of Hillsborough's Tea Party chapter, notaxfortracks.com has found an unusual ally in the Hillsborough NAACP, which formally came out against the sales tax plan Friday.

Tampa Bay was ranked as the 15th worst U.S. region for traffic in a 2016 study, with 52 percent of commuters battling gridlock during afternoon rush hours, according to a study by the navigation and mapping company TomTom.

The MPO plan analyzed 17 potential road capacity projects that could be funded with the new tax and forecast they would result in about 30,000 fewer hours of traffic delay every day.

But the expected influx of residents is a major challenge.

Commuters need to allow 50 percent more time in rush hours in Florida's major urban areas, including Tampa, a 2013 Florida Department of Transportation analysis found.

Without spending on congestion management, that number is expected to grow to 150 percent by 2040, said Beth Alden, the MPO's executive director.

"Tampa is growing up into a big city, and having less congestion with our growth rates is not realistic," she said.

Some of the biggest traffic challenges for Hillsborough lie in south County, which Merrill described as "a disaster." Dozens of subdivisions have been approved in Riverview and areas farther south with little accompanying improvements to roads and few alternatives to driving.

With the new tax, the county's bus agency would add new routes in that area.

Hillsborough Commissioner Les Miller said the county will need to make changes in how it approves future land developments.

"If this passes, we know we're in for a brand new day in Hillsborough County," Miller said. "If we do nothing, we are in a world of trouble."

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