A measure to double Alameda County's transportaton sales tax to a full 1 cent was narrowly failing at the polls, but the outcome could change when a large number of mail-in ballots are counted.

With all precincts reporting, Measure B1 garnered 65.5 percent of the vote -- short of the 66.6 percent or two-thirds majority needed to pass, according to unoffical results Tuesday.

There are many ballots yet to be counted -- mail-in ballots and absentee ballots turned in at polling places Tuesday.

Alameda County election officials said the uncounted ballots could amount to as much as 25 percent of the total votes.

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The measure would make the sales tax permanent and would raise $7.8 billion over 30 years to boost spending on roads, freeways, public transit, bicycle and pedestrian improvements, and transit-oriented developments.

Among its many projects, the tax increase would fund a $400 million down payment on a BART rail extension to Livermore.

The Alameda County Transportation Commission said the sales tax increase is needed to ease traffic congestion, help beleaguered transit agencies restore service cuts, and provide a more reliable source of transportation funding after years of state and federal cuts.

The biggest share of the money -- 48 percent -- would go to public transit and paratransit programs. A 30 percent share of the money would go to maintain local streets and roads.

Opponents said the sales tax is too much, and imposes too much of a burden on Alameda County residents.

The passing of Measure B1 would make Alameda the first county in Northern California to boost its transportation sales tax to a full 1 cent.

The current tax rate in most Alameda County cities is 8.75 cents for $1 of taxable sales -- or 8.75 percent. The rate would rise to 9.25 cents if Measure B1 passes -- bumping up the current half-cent for transportation to one-cent.

The Alameda County Transportation Commission, a 22-member panel of elected city, county and transit district officials, developed the tax increase proposal.

The tax increase was supported by many business, labor, and environmental groups. All 14 city councils and the Board of Supervisors in Alameda County voted to put the measure before voters.

Among the measure supporters were the Bay Area Council, chambers of commerce in Dublin, Pleasanton, Livermore and Oakland, and the Oakland-based TransForm, an environmentally oriented advocacy group for public transit.

There was little organized opposition to the measure.

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