(CNN) -- Gas prices tumbled more than 15 cents during the past two weeks, to $2.87 per gallon of self-serve regular, a national survey said Sunday.

Trilby Lundberg, publisher of the "Lundberg Survey," attributed the 15.42-cent drop in surveys performed on August 25 and August 11 to "more than sufficient gasoline supply" and a modest softening in crude oil prices in recent days.

"It's the largest drop since last September, when the country was coming off gasoline shortages caused by Hurricanes Katrina and Rita," said Lundberg, whose survey tallies prices every two to three weeks at about 5,000 gas stations in all 50 states.

Historically high prices had already slowed or halted the growth in demand for gasoline, she said.

Demand will drop further next month, she predicted, as Americans finish their summer vacations.

She predicted further price drops "if there is no new threat to oil supply, let alone any hurricane damage to refineries."

Gasoline stocks, imports and use of refining capacity are all up, she said.

Drivers in Des Moines paid the least, at $2.60 per gallon; drivers in Honolulu the most, at $3.23. Chicago led the Lower 48, at $3.20.

Here are some other cities:

Atlanta: $2.73

Boise, Idaho: $2.99

Boston: $2.91

Dallas: $2.72

Portland, Ore.: $2.91

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