CAMARILLO, Calif. (CNN) - The average price of gasoline increased 5 cents a gallon over the last two weeks, according to a national survey released Sunday.

Americans paid an average of $1.7212 a gallon, less than 1 cent below the all-time high of $1.7271 on May 18, 2001, the Lundberg Survey said. The numbers were not adjusted for inflation.

A year ago, the nationwide average price for a gallon of gas was $1.20.

The survey looked at prices at more than 7,000 gas stations in all 50 states and Washington, D.C., Friday.

The lowest price nationwide was in Atlanta, where a gallon of self-serve regular cost $1.51. The highest was in the San Francisco Bay area, where a gallon cost $2.10.

Though the average price of a gallon of gas continues to rise, the rate of increase has slowed after some fairly dramatic hikes earlier this year, the survey found. Average gas prices soared 11 cents between Jan. 24 and Feb. 7, and 7 cents between Feb. 7 and Feb. 21.

Related data  Gas prices state by state

Part of the reason for the slowed rate of increase can be attributed to the weather, survey publisher Trilby Lundberg said. As the winter season ends, U.S. oil refineries are making less heating oil, which comes from crude oil. The reduced demand for heating oil has freed up more capacity for gasoline production, she said.

The overall increase was affected by sharp spikes in prices on the West Coast caused by California's demands that refineries reformulate gas to meet environmental standards.