(CNN) -- Gasoline prices have jumped another 12 cents over the past two weeks, according to a survey published Sunday. The average price of a gallon of self-serve regular is now $3.81, the Lundberg Survey found. It was less of a jump than the previous two weeks, when the price climbed 18 cents, said publisher Trilby Lundberg. The average price found by the Lundberg Survey has jumped 30 cents in four weeks. While crude oil is the biggest factor, events on the U.S. West Coast are helping keep retail prices up as well, she said. "Gasoline prices would like to peak soon because there is too much gasoline supply versus demand," Lundberg said, "except on the West Coast due to refinery maintenance projects.' The higher prices on the West Coast "attract gallons from elsewhere in the country," impacting supply nationwide, she said. "So it creates a slightly tighter gasoline market for now." "When those maintenance activities come to an end in the next few weeks, that will mean even greater gasoline supply versus demand," Lundberg said. But the future of crude oil prices, the dominant factor in determining gas prices, remains unclear, she said. "Of course, all of the real or perceived threats to Middle East oil continue to be in play, so it can't be known." The gas price, a hot-button issue in the U.S. presidential race, is up 31 cents from the average a year ago, Lundberg said. The Lundberg Survey tabulates prices at thousands of gas stations nationwide. It latest figures were collected Friday. The city with the lowest average price in the latest survey was Denver, at $3.36. The highest was Los Angeles, at $4.35 Here are prices in some other cities: -- Boston, $3.78 -- Philadelphia, $3.76 -- Atlanta, $3.72 -- Chicago, $4.08 -- Indianapolis, $3.77 -- Houston, $3.66 -- Las Vegas, $3.77 -- Seattle, $4.00