A measure to increase Alameda County's transportation sales tax was defeated after a partial recount failed to reverse its razor-thin loss at the November polls.

The Alameda County Transportation Commission announced Wednesday it was conceding defeat of Measure B1, which would have doubled the sales tax to 1 cent.

The tax increase would have raised $7.8 billion over three decades for roads, freeways, transit and trails. It would have restored public service transit cuts, funded a backlog of road repairs and contributed $400 million for a BART rail extension to Livermore, among other projects.

The measure was supported by 66.53 percent of the votes, falling less than 800 votes shy of reaching the 66.67 percent needed to pass.

The final tally: 350,899 yes votes, and 176,504 no votes.

"We wanted to leave no stone unturned. We now see no value in continuing the recount," said Art Dao, executive director of the agency that oversees the transportation sales tax. "We are encouraged that 66.53 percent of the voters supported the measure."

Dao said his agency commissioned the county registrar of voters to recount ballots in many Berkeley precincts, but the results on Tuesday netted an increase of only seven "yes" votes -- not enough to justify a full recount.

The commission decided Tuesday night to pull the plug on the recount and accept the results. The agency spent about $8,000 on the recount, Dao added.

While the tax increase failed, the transportation commission hasn't given up on trying to raise the tax.

Some commissioners have said they favor going back to voters with a similar tax increase in the future but eliminating a controversial feature that would make the tax permanent.

Some critics said the commission was being arrogant and unresponsive in asking for a tax in perpetuity.

A previous half-cent transportation sales tax ballot measure in Alameda County had a sunset clause that set an expiration date unless the tax was renewed by voters.

Measure B1 would have not required voter renewal, although a list of new transportation projects to be funded would have been placed before voters at intervals about every 20 years.

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