One key input to the model is the total vehicle miles traveled (VMT) in the US over the 21^st century.  While improved fuel economy with technologies such as hybrid vehicles can cut down air pollution and oil consumption per mile, if there are more cars on the road traveling farther each year, then pollution could get worse despite improved vehicle efficiency. Historically, VMT has been increasing steadily due to several factors:

• Increasing population
• Increasing percentage of cars per person
• Increasing miles traveled by each car

For this computer model, we assume that:

1. US Population increases from 300 million today to 560 million by 2100, which is consistent with US census projections through 2050.
2. The number of cars per person increases modestly from 0.82 cars per person today to 0.92 cars per person by the end of the century; this is much slower than historical growth rates, which, if continued, would mean more than one car per every man, woman and child in the US.
3. The annual increase in miles traveled per car will slow considerably from historical growth rates, but will still increase from an average of 11,500 miles per year to 16,000 miles per year by 2100.

With these assumptions, the total VMT for all US drivers increases from 2.7 trillion miles in 2007 to 8.3 trillion miles by 2100: