not usually included in the green vehicle category, but Personal rapid transit (PRT) vehicles probably should be. All vehicles that are powered from the track have the advantage of potentially being able to use any source of electric energy, including sustainable ones, rather than requiring liquid fuels. They can also switch regenerative braking energy between vehicles and the electric grid rather than requiring energy storage on the vehicles. Also, they can potentially use the entire track area for solar collectors, not just the vehicle surface. The potential PRT energy efficiency is much higher than that which traditional automobiles can attain. Solar vehicles are electric vehicles powered by solar energy obtained from solar panels on the surface (generally, the roof) of the vehicle. Photovoltaic (PV) cells convert the Sun's energy directly into electrical energy. Solar vehicles are not practical day-to-day transportation devices at present, but are primarily demonstration vehicles and engineering exercises, often sponsored by government agencies. However, some cities have begun offering solar-powered buses, including the Tindo in Adelaide, Australia. Wind-powered electric vehicles primarily use wind-turbines installed at a strategic point of the vehicle, which are then converted into electric energy which causes the vehicle to propel. Animal powered vehicles Horse and carriage are just one type of animal propelled vehicle. Once a common form of transportation, they became far less common as cities grew and automobiles took their place. In dense cities, the waste produced by large numbers of transportation animals was a significant health problem. Oftentimes the food is produced for them using diesel powered tractors, and thus there is some environmental impact as a result of their use. Human powered vehicles Human-powered transport includes walking, bicycles, velomobiles, row boats, and other environmentally friendly ways of getting around. In addition to the health benefits of the exercise provided, they are far more environmentally friendly than most other options. The only downside is the speed limitations, and how far one can travel before getting exhausted. Benefits of green vehicle use Environmental Vehicle emissions contribute to the increasing concentration of gases linked to climate change. In order of significance, the principal greenhouse gases associated with road transport are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Road transport is the third largest source of greenhouse gases emitted in the UK, and accounts for about 27% of total emissions, and 33% in the United States. Of the total greenhouse gas