SAN DIEGO -- A supporter and an opponent of a transportation tax extension agreed at a debate Wednesday that the region's corridors are clogged and that residents need relief.

But they differed sharply on whether extending the region's transportation tax, known as TransNet, could deliver that relief.

At a debate in San Diego, the two sides squared off over Proposition A, a ballot initiative that would extend the half-cent sales tax for 40 years. The tax expires in 2008.

"We can't sit back and wait and hope this gets better," said Mitch Mitchell, a proponent of the extension and the vice president of public policy and communication for the San Diego Regional Chamber of Commerce. "People are moving here and we have to move people."

But opponent Carolyn Chase said so far the tax, which was first levied in 1988, has raised billions of dollars without easing congestion.

"It hasn't worked out like they promised," she said.

Chase is a spokesperson for a group called Taxpayers for Better Transportation Planning, which opposes the extension.

The extension is expected to raise $14 billion in revenues, which would be split into rough thirds among freeway improvements, local roads and public transportation projects.

The forum, a luncheon hosted by the Society of Marketing Professional Services, was the first to feature a debate between an opponent and a proponent of the extension. The society is an organization of business people in the architectural, engineering, planning and construction fields.

Mitchell and Chase delivered separate speeches to an audience of about 75. The two also sparred from time to time.

Mitchell told the group gathered at a Mission Valley hotel that TransNet may not be perfect, but that in a region with abysmal gridlock, a growing population and stratospheric home prices, the tax extension and the billions of dollars worth of projects it would fund are desperately needed, Mitchell said.

He singled out specific projects he said were critical, including some $2.69 billion in improvements slated along the Interstate 5 corridor.

He said the region could have planned better for its transportation needs, citing the lack of a light-rail system along Interstate 15 and the death of a proposed north-south highway in North County as examples.

Chase, a San Diego planning commissioner and the chairwoman of the political committee for the local chapter of the Sierra Club, criticized what the TransNet tax has done to date, saying cost overruns and poor planning have delayed critical projects and kept congestion at the status quo.

"If they were such great traffic planners, why didn't they solve the problems on (Interstates) 5 and 15, 10 or 15 years ago?" she asked rhetorically.

She also pointed to studies done by the association of governments that show that even after billions of dollars are spent on transportation improvements, commute times for most drivers will not significantly improve.

The measure needs to be approved by two-thirds of voters to pass.

Contact staff writer Katherine Marks at (760) 740-3529 or kmarks@nctimes.com.