ARCATA, Calif. --- The latest Lundberg Survey found that gas prices rose more than 9 cents over the past two weeks, resulting in a national average of $3.20 per gallon for self-serve regular, CNNMoney.com reported.

The survey compared prices compiled March 7 with those compiled Feb. 22 at about 5,000 gas stations nationwide. The highest prices were in San Francisco ($3.58) and the lowest in Cheyenne, Wyo. ($2.99).

Lundberg Survey Publisher Trilby Lundberg warned that gas prices aren't expected to fall anytime soon.

"We should be ready to absorb even higher prices, and quickly," she said. She attributed the higher prices to a number of factors, including OPEC's decision not to raise production, growing world demand for oil, and the "mileage penalty" associated with government-mandated increases in the use of ethanol in gasoline.

"As the mandate brings more ethanol into the pool, the mileage penalty grows," she said. Last year, when 4.3 percent of gasoline was ethanol, vehicles got 1.4 percent less miles per gallon than they would have on non-ethanol gasoline, she said. This year, with a mandate of 6.3 percent, the average MPG will drop by 2.1 percent, she said.

Moreover, Daylight Saving Time will spur added demand throughout the summer, and could result in price increases of "at least 20 to 30 cents at the pump, perhaps by mid-April," Lundberg said.