Voter turnout was 15 percent higher for the Nov. 3 election in San Mateo County than it was in November 2013, the last off-year election that can be considered as a fair comparison, county election officials say.

Of the 357,191 registered voters mailed ballots this time, 105,325 returned them, mostly by mail, according to the final semi-official tally released by the county Elections Office on Nov. 12. That's a turnout of 29.5 percent compared to 25.4 percent in 2013, according to Elections Office records.

The principal difference this time, according to Jim Irizarry, San Mateo County's assistant chief elections officer: the 2015 election was held by mail. Accommodations were made for in-person voting, but the county mailed ballots to all registered voters in a package that included return envelopes with prepaid postage, Mr. Irizarry said.

This method of voting sending mail-in ballots to all registered voters is being tested in two California counties: San Mateo and Yolo, which are authorized by state law to hold three such off-year general elections between 2015 and 2017. The results will be reported to the state Legislature, which will decide whether to adopt the method statewide.

San Mateo County's Elections Office officials will prepare a report for the state Legislature and the Secretary of State's office, Mr. Irizarry said.

Voting by mail

In all, 102,710 voters, about 98 percent, mailed their ballots (or dropped off their mail-in ballots), while 2,415, about 2 percent, voted in person at 32 universal polling places and two election centers in the county, according to Elections Office data. In the 2013 election, those numbers were 76 percent using mail-in ballots and 23 percent voting in person.

For the Elections Office, there were clear benefits to so many mailed-in ballots, Mr. Irizarry said. By their nature, elections are intense, laborious and, with 357,000 registered voters, logistically complex, he said.

On Election Day, in a typical election with a polling place in every precinct, the Elections Office deploys 1,700 to 1,800 poll workers and 1,400 voting machines, he said.

For this election, the number of voting machines was reduced to a few for each of the 32 universal polling places (including one polling place each in Menlo Park, Woodside, Portola Valley and Atherton), and the number of poll workers was reduced to 135.

While ballots differ significantly from one community to another and from one public jurisdiction to another, each of the universal polling stations could provide an accurate ballot to any voter in the county.

The county spent $72,000 to mail 357,000 ballots, and $52,000 on paying the postage for the 105,000 ballots mailed back, he said.

Adding to the overall efficiency was lead time, Mr. Irizarry said. By the time Election Day rolled around, the Elections Office had been receiving and processing ballots in a database for 29 days. On Election Day and night, he said, it "was not even close to the normal level of intensity."

"It's been a fun election," he said. "Definitely the preferred method of voting is all-mailed-in ballot."