There’s a downside to more fuel-efficient cars—people drive them more, which cancels out the savings in both money and emissions. By effectively making a resource less scarce, we’re prone to value it less.
In Britain, roughly 20% of the energy saved by more efficient cars is wasted, as drivers make more trips. A new study, from the University of Sussex, took data from 1970 to 2011 and discovered this "rebound effect." This effect happens in many areas. For instance, says the report, "We know that insulation of housing encourages people to enjoy warmer homes, rather than taking all the benefits in the form of lower bills."
It’s not the efficiency itself that causes people to drive more, but the cost savings. In fact, when the study looked at the figures purely for efficiency, across 108 models of car, they found little correlation between extra use and more efficient cars. When they looked at it from a cost point-of-view, though, the researchers found "a direct rebound effect in the range 9% to 36% with a mean of ~19%." It’s important to note that here, "cost" consists of actual price changes, combined with lowered costs thanks to greater fuel efficiency.
Deducing the true rebound effect from decades worth of data required some work on the numbers. For instance, if people planned to do a lot of long-distance travel, wouldn’t they chose a more fuel-efficient car? This could bias the results, inflating the rebound effect figure. The most common method for estimating the rebound effect is to hook up the distance traveled with the cost per mile, with a lot of statistical math on top.
These figures match up with those found in U.S.-based studies, although this result wasn’t certain. In fact, the unique conditions of U.S. car use—low fuel prices, high car usage, low vehicle efficiency and low population density—should make U.S. studies a terrible model for the rest of the world.
This unexpected match-up suggests that things like "differences in land use patterns, car dependence, and travel costs have little influence on the relative size of the rebound effect," say the authors.
That’s not to say that more efficient cars are bad: A 20% rebound means 80% non-rebound. That’s a good saving—clearly people are burning a lot less fuel today than they were 40 years ago. And things may be set to change a whole lot more. The authors speculate that "growing evidence for ‘peak car’ implies that improvements in vehicle fuel efficiency may have much less impact on distance travelled than in the past." That is, there will simply be fewer cars on the roads in future, which will itself lead to a big drop in fuel consumption, along with all the other benefits of fewer automobiles.