From CNN Correspondent Jim Hill

LOS ANGELES (CNN) -- As the summer vacation season unofficially begins in the U.S., the cost of a fill-up crosses the mind of many a motorist. The news is mostly good: Pump prices in recent months have been the lowest in decades.

They have risen with the season's start, but not by much.

"Demand is increasing in the later part of May with Memorial Day weekend kicking it off, " explains AAA's Jeffrey Spring. Increased summertime demand means slightly higher prices.

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In California, where many drivers live and others like to vacation, prices will be higher than the national norm -- from ten-to-30-cents a gallon. Some observers say heavy demand is one reason -- and another is California's law requiring cleaner-burning, more highly refined gas.

"That gasoline takes about the equivalent of a billion pounds of pollutants out of the air every year," said Anita Mengels of the Western States Petroleum Association, "so it might cost a little more. But I think most Californians might agree that it's worth it to have clean air."

Not everyone agrees. One lawmaker wonders if Californians are being gouged at the pump -- and has asked the Federal Trade Commission to investigate.

"We believe there's a lack of competition now because of mergers and because there are so few independent retailers," said Sen. Barbara Boxer, D-California, "and frankly we're finding some harassment of retailers who want to lower the prices."

In spite of it all, a record number of people are expected to vacation this summer -- and as usual, the vast majority of them will fill-er up and go by car.