We have constructed four major scenarios to represent a plausible mixture of alternative vehicles and fuels being considered by the major automobile companies at this time:

1. A base case with conventional vehicles plus a growing percentage of gasoline-powered hybrid electric vehicles (HEVs), the HEV scenario.
2. A scenario the same as #1, but adding increasing numbers of gasoline-powered plug-in HEVs over time, the gasoline PHEV scenario.
3. A scenario the same as #2, but with biofuels such as cellulosic ethanol replacing gasoline in the PHEVs, the biofuel PHEV scenario.
4. A scenario the same as #3, but with hydrogen-powered fuel cell electric vehicles (FCEVs) gradually replacing all the other vehicles over time in proportion to their numbers in #3 each year, the FCEV scenario.

We also analyzed two secondary scenarios:

1. A scenario the same as #4, but with hydrogen-powered internal combustion engine HEVs replacing the FCEVs.
2. A scenario the same as #4, but with battery-powered electric vehicles (BEVs) replacing the FCEVs.

Finally, we also have shown a “Business-as-usual” reference case with all gasoline internal combustion engine vehicles (no hybrids).  This scenario will not happen, since HEVs are already entering the fleet in large numbers, but we use it solely as an illustration of the worst possible case in terms of pollution and oil consumption.

These scenarios are not meant to predict what vehicles will be on the road in the future, but they are constructed with careful thought about what might be possible given appropriate government and industry actions to stem the triple threat of climate change, oil dependence and urban air pollution.