ATLANTA (CNN) - Gas prices remained flat over the past three weeks, at an average of $2.93 per gallon for self-serve regular, a national survey said Sunday.

Increased supply, stagnant demand and relatively stable crude oil prices combined to keep prices steady, said Trilby Lundberg, publisher of the Lundberg Survey. Her survey tallied prices at about 5,000 gas stations in all 50 states on May 19 and again on June 9.

"Even though we consume more in June than we do in May, increased refining capacity coming back from pre-summer work projects, plus heavy imports of gasoline, has increased gasoline supply," she said. Work projects included routine maintenance as well as repairs to refineries damaged during last summer's hurricanes along the Gulf Coast.

Though gasoline demand typically grows from year to year, that has not been the case in recent weeks, she said. And the cost to refiners of carrying out a federal mandate to blend ethanol into gasoline supplies had been factored into gas prices by late April, she said.

Drivers in Wichita, Kan., paid the least, at $2.66 per gallon, the survey found. Drivers in Honolulu paid the most, at $3.27.

Here are some other cities: Denver: $2.77; Atlanta: $2.90; Houston: $2.92; Portland, Ore.: $3; Boston: $3.01; Washington: $3.07; San Francisco: $3.21; Pittsburgh: $2.81.

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Special Report: Gas Crunch: 2006