"Gas prices display what economists call an asymmetric relationship. They do tend to go up faster than they fall," says Walden, a specialist with the North Carolina Cooperative Extension Service at N.C. State University. "The question then becomes, 'Why?'

"Now many people think this is all contrived by the oil companies. But really a careful analysis, I think, reveals a much less sinister reason," he says. "Here's the explanation: After gas prices have been high and they begin to come down, drivers begin to increase their demand -- in part because they want to stock up on gas in case the prices go up again.

"Now those increased purchases again increase demand, and that causes the price decline to slow. So this tends to be the reason given by economists, but, of course, there are competing explanations."