MIT professors Jeroen Struben and John Sterman have developed a computer model to tell us what a a few minutes of observation in area outside of the likes of Newport Beach could tell you: it will take 15-20 years for half the US vehicle fleet to switch over to new vehicles like hybrids.
The reason for this is simple economics. There are a lot of people who buy new cars every couple of years, but those cars that are replaced don't just disappear. Older cars are handed down the economic ladder and modern cars often last 15-20 years. There are tens of millions of people in this country who can't afford even $25,000-30,000 for a new car and rely on being able to buy older cars for a few hundred dollars.
In order to make a dent in fuel consumption, more needs to be done to enhance public transit and promote the use of it. We also need to get people to change their habits when they do drive by consolidating trips and carpooling. As fuel prices go up, lower income drivers will migrate to more efficient vehicles as well just because of the operating cost savings.
[Source: New Scientist Tech]