A ballot measure to double Alameda County's transportation sales tax to 1 cent was still trailing Wednesday with only a few thousand provisional ballots left to be counted, election officials reported.

Support for Measure B1 climbed up to 66.34 percent of votes -- about 0.33 of a percentage point short of the 66.67 percent, or two-thirds, needed to pass.

It appears there are some 8,000 or more votes yet to be counted.

About midday Wednesday, election officials said there were about 16,000 provisional ballots to be counted and later posted results for another 7,222 votes cast in the Measure B1 race.

Some voters who cast ballots didn't vote for or against Measure B1.

Dave Macdonald, the Alameda County registrar of voters, estimated it could take another two days to finish the labor intensive task of counting most of the provisional ballots.

Measure B1 is being closely watched in transportation circles because it would make Alameda County the first county in Northern California to collect a full 1-cent transportation sales tax.

The sales tax increase would raise some $7.8 billion over 30 years for roads, freeways, public transit, trails, bike lanes and transit-oriented development.

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