CONCORD -- Two opinion polls that found high support for a higher Contra Costa County sales tax for transportation have stirred up interest for a November 2016 ballot measure.

The tax is used to fix potholes, improve freeways, boost public transit and other measures aimed at easing traffic congestion.

For years, commissioners overseeing transportation have been reluctant to consider increasing the tax from a half-cent to 1 cent out of fear that voters would say no.

Public opinion appears to be warming, though, as the economy improves and the Contra Costa Transportation Authority does highly visible projects such as the Caldecott Tunnel fourth bore and widening of Highway 4 in Pittsburg and Antioch, officials said.

The authority commissioned polls in January and March, and each came up with consistent results: 68 percent support for extending the tax and increasing it to 1 cent. A measure needs two-thirds to pass.

"It's not easy to get a tax increase approved, but strong polls like these definitely make you think it's possible if voters clearly understand the transportation projects that would be funded," said Kevin Romick, an Oakley city councilman who chairs the commission.

Romick said results of the first telephone poll were surprising, so the commission asked for a second one, and that confirmed the results.

Also, the number opposed to a tax increase dropped from 29 percent in the first poll to 26 percent in the second poll.

Pollsters from the Oakland-based EMC Research did the survey, conducting 814 telephone interviews for the first poll and 606 interviews for the second one in March. The margin of error was plus or minus 4 percentage points.

One anti-tax leader said raising the sales tax would be a mistake, especially as other agencies consider tax increases this year in Contra Costa.

"The taxpayers in this county are overburdened with taxes," said Ken Hambrick, chair of the Alliance of Contra Costa Taxpayers. "We never see the results of this money being spent in an effective way."

Raising the transportation sales tax to 1 percent, or 1 cent per $1 of taxable sales, would boost the total sales tax rates from 8.5 percent to 9 percent in most cities in Contra Costa.

Newell Arnerich, a Danville city councilman on the transportation commission, said he thinks county residents are more receptive to a tax hike because they are reaping benefits from authority projects like the Caldecott Tunnel fourth bore.

The authority faces a challenge, however, because many people aren't familiar with the agency, which is separate from the county, he said.

"When a freeway is widened or another bore is added to the Caldecott Tunnel, the general public thinks Caltrans did it because Caltrans is well known, and the Contra Costa Transportation Authority is not," Arnerich said. "Yet many projects wouldn't get done if not for the authority and the local sales tax."

The authority partners with Caltrans on many projects and supplied $120 million in local sales tax money for the $417 million new Caldecott Tunnel fourth bore, which opened in November on Highway 24 between Orinda and Oakland.

Arnerich said the authority is developing a master plan for long-range transportation projects so it can lay out its priorities. Approval of the plan is scheduled by the end of the year.

"I want to have a clear list of transportation projects so voters can make a choice and vote them up or down," he said.

Contra Costa voters approved the transportation sales tax in 1988 as Measure C and voted in 2004 to renew it until 2034 as Measure J.

In Alameda County, voters in November will consider doubling the transportation sales tax from a half-cent to 1 cent. It would be the first county in Northern California to increase to 1 cent.

Contact Denis Cuff at 925-943-8267. Follow him at Twitter.com/deniscuff.