Click photo to enlarge
File photo.

Notice anything different on that holiday road trip this year? More drivers are on the road, and they're paying more for gas.

Bay Area gas prices have soared to their highest point in more than two years.

The average price of a gallon so far this month has been $3.28, up 76 percent from the low point in December 2008, according to AAA Northern California.

December marks the 13th time in the last 14 months that the average price has gone up compared with the prior year.

In fact, the slow and steady rise in gas prices over the past two years has put the Bay Area more than halfway back to the peak average price of $4.50 during the summer of 2008. After reaching that high, gas prices fell to about $2 through the winter amid the economic collapse. They've been creeping back up ever since.

The recent price change is particularly unusual because fuel is usually its at its cheapest this time of year. But a slow uptick in the economy, combined with a drop in supply, have turned that tradition on its head, experts said.

In San Jose, the average price Friday was $3.31, up two cents from the previous week and 11 cents from a month ago.

The average price in Oakland was $3.30, up two cents from a week ago and 11 cents from last month.

In San Francisco, the average price was $3.37, up two cents from last week and 10 cents from last month.

Already, the increase has added up. It now typically costs just less than $40 to fill up a 12-gallon tank with regular gas, up $7 from the 2009 average and $17 more than the low point two years ago.

At this rate, motorists would fork over $3.50 per gallon by the summer, when oil prices typically hit their peak. The government expects gas prices to continue to rise next year as more people get back to work and hit the roads, but experts don't think it will rise above $4 per gallon.

Tupper Hull, spokesman for the oil industry group Western States Petroleum Association, said many factors affect the price of gas.

But nearly two-thirds of the cost that drivers pay at the pump is the result of the global price of crude oil, which is used to make vehicle fuel. It's a textbook case of supply and demand: The petroleum becomes more expensive as more people fill up.

The AAA predicted overall travel to rise about 3 percent this holiday season, with more than 92 million people planning to go more than 50 miles sometime between now and Jan. 2.

More than 90 percent said they would be driving.

In California, with the various fees and taxes assigned to gas, the price at the pump is typically about 25 cents higher than the national average, and the cost is usually about a dime per gallon higher in the Bay Area than the rest of the state. Only this month has the national average reached $3 a gallon.

AAA spokesman Matt Skryja said commuters have not been complaining about the rise in gas prices the way they were in 2008, mostly because a lot of drivers don't notice prices until they hit certain milestones, such as the dreaded $4 per gallon mark.

"We are at a point right now where we've been past $3 for quite awhile, and there is no imminent concern about the $4 mark being surpassed," Skryja said.

The Associated Press contributed to this report. Mike Rosenberg covers San Mateo, Burlingame, Belmont and transportation. Contact him at 650-348-4324.

THE PRICE in SAN JOSE

$3.31

Average price of gas Friday, an increase of 11 cents from a month ago.

ONLINE: Hitting the road this weekend? Check traffic reports at www.mercurynews.com/traffic.