Actually, the ultimate green car would be no car at all. Even if we all drove zero-emission cars, it would still require expanding our vast and inefficient roadway infrastructure. In cities, the use of cars inevitably leads to congestion and polluted air. An optimal transportation system is a car-free public system of metros and high-speed intercity trains. Does this sound idealistic? Most New Yorkers are already car-free, but unfortunately, not all of them, hence they are subjected to noise and pollution. In Italy, Venice is an example of a modern city that is virtually car-free today. Central to a care-free city is a metro system that is cheap, clean, comfortable and runs 24 hours-a-day on a regular schedule (4-8 minutes intervals). If planned and funded correctly, metro stations are within walking or biking distance of all areas of the city. This enables reliable door-to-door transportation for everyone. Intracity transportation could be served by high-speed rail (see Japan or Italy). For more details see Carfree Cities.Until we have a car-free infrastructure built into our cities and towns, an interim goal would be to design a efficient car that would replace the highly ineffective ones we have today. An optimal car would be a car that is both a solar-powered, plug-in electric car and a biodiesel-powered hybrid. It would be a combination a plug-in Prius and Saab's biodiesel hybrid prototype. (Or perhaps instead of the Prius the Tesla Car). At home, the owner of the car would plug it into a solar electric system (or other renewable energy system) and charge it. This clean electric power would be used for trips under 100 miles. For longer trips, over 100 miles, the car would switch over to biodiesel fuel, just like hybrid cars today. We are assuming, in this scenario, that biodiesel pumps are widely available across the country. Well, there you have it, a blueprint for a extremely green car that performs just like the ones we have today. Oh yes, I forgot, also build the car so that it is a technical or biological nutrient.