The largest car rental company in the United States, Enterprise Rent-a-Car, announced it’s opening six “green” branches in Atlanta, Ga., where 60 percent of the available rental vehicles will be hybrids or other fuel-efficient cars. The agency said the increase in efficient vehicles is due to consumer demand. Enterprise currently has a fleet of about 4,000 hybrids out of a total 1.1 million vehicles nationwide. Rival rental agencies have also touted their own hybrid fleets. Avis said that it currently has about 2,500 hybrids, while Hertz said it aims to offer some 3,500 hybrid vehicles sometime this summer.