comScore Media Metrix ranks top 50 U.S. web properties for March 2011. Americans are exploring greener lifestyles for earth hour as gas prices climb. Warmer months induce traffic increases to travel sites. Car Rental sites grew 9 percent to 6.0 million unique visitors in March, with Enterprise Rent-A-Car Company taking the #1 spot with 3.3 million visitors (up 9 percent) and Avis Budget Group with 1.8 million (up 4 percent). Hertz came in third with 979,000 visitors, followed by Dollar Thrifty Automotive Group, Inc. with 968,000 (up 19 percent) and CarRentals.com with 934,000 (up 31 percent).