University of Michigan

Self-Driving Cars May Increase Rather Than Decrease Gas Use

May 8, 2019
Being able to work, watch movies, play games, or sleep while “driving” a self-driving car may lead people to put more miles on their cars.

A few decades ago when CAFÉ standards starting significantly increasing the average mileage U.S. cars got, one of the goals was to reduce the amount of gas they used. Instead, the amount of fuel increased as drivers got more bang for their bucks. This behavioral change was dubbed the rebound effect.

There are several predictions touting the expected benefits of self-driving cars. One of them is that cars should get even better mileage and gas use should drop.

Not so fast, according to research done at the University of Michigan. The researchers predict that the benefits of self-driving cars will likely cause vehicle owners to drive more, and those extra miles could partially or completely offset any potential energy-saving benefits that automation may provide. For example, being able to work, sleep, watch a movie, read a book, or play a game of cards with other occupants will likely induce even more travel.

Conceivably, the added miles could increase  energy consumption, a phenomenon known as backfire. “So the potential and likely increase in travel caused by self-driving cars presents a stiff challenge to policy goals for reductions in energy use,” says researcher Samuel Stolper.

“This means much higher energy-efficiency targets are required for self-driving cars if we are to keep gas use constant or lower it,” adds fellow researcher Ming Xu.

The team used economic theory and U.S. travel survey data to model travel behavior and forecast the effects of vehicle automation on travel decisions and energy use. Most previous studies of the energy impact of autonomous vehicles focused exclusively on the fuel-cost component of the price of travel. This likely resulted in an overestimation of the environmental benefits of the self-driving technology, according to the UM engineers.

In contrast, the Michigan study looked at both fuel and time cost. Their approach adapts standard microeconomic modeling and statistical techniques to account for the value of time. Traditionally, time spent driving has been viewed as a cost to the driver. But the ability to pursue other activities in an autonomous vehicle is expected to lower this “perceived travel time cost” considerably, which will likely spur additional travel.

The UM researchers estimated that the induced travel resulting from a 38% reduction in perceived travel time cost would completely eliminate the fuel savings associated with self-driving cars. And the possibility of backfire, in turn, implies the possibility of an increase in local and global air pollution.

In addition, the researchers suggest there’s an equity issue that needs to be addressed as autonomous vehicles become a reality. The study found that wealthier households are more likely than others to drive extra miles in autonomous vehicles “and thus stand to experience greater welfare gains.”

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