A study done by the Dr. Michael Sivak, the director at the Sustainable Worldwide Transportation in the University of Michigan Transportation Research Institute (UMTRI), discovered that while fuel economy has improved greatly from 1970-2010, the improvement is greatly undercut due to the amount of people who drive more often and with less passengers in their cars. The study showed that the fuel economy for the entire fleet of light duty vehicles improved by 40%, from 13 mpg to 21.6 mpg, but the vehicle load (number of occupants) decreased from 1.9 persons per vehicle to about 1.38 persons.
Because of the decrease in carpooling, Sivak discovered that the fuel economy for the fleet of light duty vehicles has only increased by 17%. Sivak also discovered that people travel more often thanks to improved fuel economy in vehicles. This is due to the rebound effect. To determine the nominal reduction of fuel consumption for a vehicle, you have to take the % increase of fuel economy and subtract 10% due to the rebound effect. An example would be a 20% increase in fuel economy results in an 18% reduction of fuel consumption.
Sivak also notes that it takes a long time for changes in the fuel economy to apply to the entire fleet of light duty vehicles, so an 18% reduction in fuel consumption would only result in 1% reduction of fuel used by the entire fleet. This brought up a discussion between researchers about whether or not there should be an increase on fuel tax to reduce the amount of vehicular distance traveled.
Another way to improve the fuel economy is if people carpooled more often. The vehicle distance traveled has increased by 155% from 1970-2010. If the vehicle load is increased by 20%, from 1.38 persons per vehicle to 1.66 persons per vehicle, the distance traveled would decrease by 15%, and while there would be a 1% decrease in fuel economy (due to the heavy load), there would be a 14% decrease in fuel consumption.
The amount of fuel used between 1970-2010 has increased by 53%, from 303 billion liters to 463 billion liters. As vehicles become more fuel efficient, people want to drive more often. There's also the inconvenience factor some see while carpooling. Like the LA Times put it, "People are driving alone because they still 'want the convenience of driving by themselves. They are not eager to drive an extra mile to pick up a friend.'"
[via Green Car Congress]