BOSTON – Massachusetts state officials are considering allowing solo drivers of hybrid cars to use high-occupancy vehicle lanes normally reserved during rush hour for vehicles carrying at least two people.The goal is to encourage more people to buy hybrid vehicles. Arizona, California, and Virginia already have similar policies, though only Virginia allows hybrids in high-occupancy lanes.The Massachusetts policy, under discussion by officials of the Commonwealth's transportation and environmental agencies, would apply to the movable six-mile zipper lane on the Southeast Expressway during morning and evening rush hour, and the 2-mile high-occupancy lane on Interstate 93 southbound from Somerville to the Bunker Hill Bridge.The high-occupancy lanes, as well as new two-person-minimum lanes that are part of the Big Dig roadway network, are usually wide open during rush hour and cut commuting time dramatically, while other travel lanes are typically bumper to bumper.The move would be the latest environmentally friendly initiative by the Romney administration, which two years ago promised to get rid of most sport/utility vehicles in the state fleet. While campaigning, Governor Mitt Romney also suggested charging higher registration fees for cars with low gasoline mileage, though that idea has not been put into effect.But allowing solo drivers of hybrid vehicles into high-occupancy lanes has not gone entirely smoothly where it has been tried. In California and Virginia, many commuters complained that the hybrid vehicles were wildly popular and that people needed no further incentive to buy them. A Virginia task force found recently that the hybrid vehicles were clogging high-occupancy lanes.In California, some car makers balked when the state wanted to limit high-occupancy access to hybrid vehicles with fuel efficiency of 45 miles a gallon or better, which excluded some models, such as the Ford Escape sport/utility vehicle. "That led to a lot of confusion," said Elroy Garcia, spokesman for the Alliance of Automobile Manufacturers. "It was limiting hybrids at a time when manufacturers are expanding the hybrid options for consumers."There is also the matter of federal approval for such policies, which has not yet been granted by Congress. A standard rule, which would become part of Federal Highway Administration guidelines, has been suggested in different bills, including an energy bill last year and the transportation reauthorization bill, which continues to languish.The policy is on hold in California and Arizona. In Virginia, solo drivers of hybrid cars are permitted to use the high-occupancy lanes even without federal approval.Massachusetts would wait until a national policy is set before opening the high-occupancy lanes to hybrid owners, state officials say. In addition, the state would have to distribute markings for the vehicles that are easily identified by the state troopers stationed to monitor the lanes.