Driverless cars like those being developed by Google, Tesla and other manufacturers promise to save energy, reduce traffic and make life on the roads safer and happier for drivers.
But new research shows that too much of a good thing may result in clogged highways and wasted fuel, as people ditch public forms of transit for the comfort of their own personal cars.
"If you have self-driving cars, you can get work done in your car and you have the advantage of being in your own personal space," said Zia Wadud, associate professor of engineering at the University of Leeds. "You could see a potential switch from other transport modes. In the United States, it's probably aviation or in the Washington to New York corridor, it could be rail."
Wadud and colleagues at the University of Washington and Oak Ridge National Laboratory published a study today in the journal "Transportation Research Part A" that looked at the energy efficiency pros and cons of automated and driverless vehicles.
The study identified these energy-saving benefits of self-driving cars, depending on how many people use them:
More efficient computer-directed driving styles (0 percent to 20 percent reduction in energy use)
Improved traffic flow and reduced jams because of coordination between vehicles (0 percent to 4 percent reduction)
"Platooning" of automated vehicles driving very close together to create aerodynamic savings (4 percent to 25 percent reduction)
Reduced crash risks mean that cars can be lighter (5 percent to 23 percent reduction)
Fewer high-performance, gas-guzzling, hot rods (5 percent to 23 percent reduction).
But the study also predicts that energy use will go up 5 percent to 60 percent as people switch to highly automated cars in situations where they would have previously taken alternatives like trains or planes.
Wadud says his team made predictions based on economic and behavioral models of individuals' energy use. But he was careful to say that the future is not easy to see. The team came up with four future scenarios.
In one scenario, the efficiency benefits do not take place, Wadud said via Skype. "That is a nightmare scenario. Generally speaking, if we have increasing travel demand and improvement in efficiency, then there will be slight reduction in energy demand."
Wadud said that cars can save a lot of energy and fuel without becoming fully automated. The important thing is that they are able to talk to each other. This will help them drive closer together without bumping into each other, and thereby take advantage of the aerodynamic drag effect.
"The majority of energy efficiency benefits can take place at lower levels of automation," he said. "You do not need self-driving cars."
The study also found that people who usually don't drive as much -- such as the elderly or those with disabilities -- might start driving more with automated vehicles. That could push down energy savings by as much as 10 percent.
The electronic demands of these new cars -- with more touch screens and TV monitors, might require up to 11 percent more energy to run as well.
Jeff Gonder, a transportation systems analyst at the National Renewable Energy Laboratory in Golden, Colo., said the energy benefits of driverless cars are still a big question.
"People are going to do what is most convenient for them, Gonder said. "If there are more cars on the road, but the train is faster, they will take the train. If they have trouble getting to the train and it adds a lot to their journey time, and can have an inexpensive automated vehicle chauffeur them door to door. They might do that."