(CNN) -- U.S. gas prices increased over a two-week period, but on average stayed below the $3-a-gallon mark.
The latest Lundberg Survey of cities in the continental United States, conducted Friday, showed the national average price for a gallon of self-serve unleaded gasoline at $2.99, an increase of 7.6 cents from the last survey two weeks earlier, survey publisher Trilby Lundberg said.
What had been a steady rise in oil prices retracted slightly, making prices higher, but not as high as they could have been, Lundberg said.
Compared with a year ago, drivers are paying quite a premium at the pump, she said. A year ago, a gallon of gas costed 39 cents less than it does today.
"That's certainly a pinch to consumers," Lundberg said.
Crude oil prices will continue to be the biggest indicator of gas prices because other factors that affect price, such as supply, have been non-factors. There is no shortage of supply of gas in the United States.
"Even a slight move in crude oil can, at this time, quickly move the price of gasoline," Lundberg said.
The Lundberg Survey sampled prices at about 2,500 gas stations. The highest average price in the continental United States was in San Francisco, California, at $3.29 per gallon. The lowest price was in Denver, Colorado, at $2.68.
Average per-gallon prices in other cities:
-- Atlanta, Georgia: $2.91
-- Boston, Massachusetts: $3.08
-- Chicago, Illinois: $3.21
-- Houston, Texas: $2.78
-- Miami, Florida: $3.01
-- Seattle, Washington: $3.14