Whilst electric cars do not pollute when they are driven, they need to have their batteries recharged.  This uses electricity produced from a power station; most power stations are coal fired and thus emit greenhouse gases.

Whilst hydrogen cars produce only non-polluting water vapour as exhaust gases, the hydrogen they use has to be obtained somehow. This usually involves fossil fuels somewhere along the line, such as in producing the electricity to power the electrolysis of water to make hydrogen; or, more likely these days, in reformers where hydrogen is made from hydrocarbons such as methanol.

Biodiesel can be made from waste vegetable oils ( Make your own biodiesel) but the vast majority is made from palm oil. It is now becoming so popular across Europe that vast tracts of rainforest in Malaysia and Indonesia are being cleared for oil-palm plantations. This is also happening in Brazil to make space to grow the sugar cane needed for processing into ethanol for alcohol powered cars. Thus the world is losing even more rainforest, releasing carbon dioxide into the atmosphere as the land is cleared. The end result is fewer trees and green plants to take up the carbon dioxide that is already in the atmosphere.

Solar powered cars are the only completely green option but they are still at the concept stage. See the World Solar Challenge website for information on the cars in the annual 3,000 km race for vehicles powered by sunlight alone. The film Race the Sun starring Halle Berry and James Belushi tells the story of a group Hawaiian High School Advanced Level Physics students who designed, built and ran a car in this race.