For three years, Los Angeles has let some hybrids and other vehicles that get more than 45 miles per gallon park for free. Bruce Gillman, a spokesman for the Los Angeles Department of Transportation, said the city offers free parking to any vehicle with the California Clean Air Vehicle decal. The New York City Council has discussed a similar idea: letting hybrid drivers park free at city meters. Houston has considered giving preferential parking to hybrids at the city's airports, but Cris Eugster, chief sustainability officer for Mayor Bill White, says it's not clear whether that actually provides incentive for motorists to purchase hybrids. 'We'd rather lead by example,' he says, noting that White drives a Prius. Hybrid vehicles pair internal combustion and electric motors, using the electric motor at low speeds and in stop-and-start driving, and the internal combustion motor at cruising speeds. Houston recently bought its 500th hybrid vehicle for the city's fleet, and the Metropolitan Transit Authority has committed to buying about 100 hybrid buses a year for a total of 450 by 2011. Several states, including California and Colorado, allow some hybrids in high-occupancy vehicle lanes with just a single occupant, but Houston has rejected the idea.