Gasoline passed $2 a gallon in 2004 and $3 a gallon last year without U.S. consumers changing their driving habits, but now that the price has breached the $4 mark, drivers have begun to conserve. "I think we've reached a tipping point," said economist James Hamilton at the University of California, San Diego. Columbia Business School economist Geoffrey Heal agrees there has been a behavioral shift in energy consumption. "If enough people like you ride the bus, you'll be more willing to ride the bus," he said.

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