Julia Roberts has joined fellow celebrity environmentalists like Willie Nelson in an effort to prompt US education bosses to fill up school buses with alternative fuels. The actress has signed on as a spokeswoman for Dallas, Texas-based biodiesel producer Earth Biofuels, and she'll also chair the company's new advisory board as it promote an environmentally friendly alternative fuel. Roberts admits she jumped at the chance to get involved with the campaign after considering the harmful effects pollution may have on her 20-month-old twins, HAZEL and PHINNEAUS, in the years ahead. She says, "It's very important that we expand our use of clean energy and make a long-term commitment to it. "Biodiesel and ethanol are better for the environment and for the air we breathe." The Oscar winner wants to start her new campaign by concentrating on promoting the use of ethanol and the soy derivative biodiesel in school buses.