To evaluate the societal benefits of alternative vehicles, we have developed a set of computer programs to model the likely greenhouse gas pollution, oil consumption and urban air pollution over time. Other organizations have compared emissions or oil use at a fixed point of time for some (but not all) alternative vehicles. But in the real world, many parameters will be changing with time as new vehicles are introduced into the fleet, including (among others):
 Number of vehicles on the road
 Number of miles each vehicle travels per year
 The fuel economy (mpg) of each type of vehicle
 The fraction of alternative vehicles sold each year
 The sources of electricity, biofuels, and hydrogen
We have written a dynamic computer model to take into account all these variables over the entire 21^{st} century. As with any model, the results depend on the assumptions.
We have constructed four major alternative vehicle scenarios that estimate the combination of regular cars and alternative cars sold each year. These scenarios then allow us to explore the societal attributes over time (greenhouse gases, oil consumption, urban air pollution, and total societal costs) for each combination of alternative vehicles. In each case we add these alternative vehicles gradually over time to simulate the slow, realworld introduction of new technology.
