The future of transit very well may be driverless cars, which promise to save us not just from traffic jams and unwieldy DMV lines, but perhaps even our environmental woes: New research published yesterday in Nature Climate Change finds that, by 2030, self-driving taxis could produce up to 94 percent less greenhouse gas emissions per mile than the conventional vehicles jamming the roads today.
Previous studies on the energy savings of autonomous cars focused on things like platooning, where the driverless vehicles roll out in aerodynamic convoys of wirelessly connected cars that break and accelerate in perfect unison with the lead car. But the emissions reductions seen in the new study go above and beyond those savings.
"It was very surprising that we could get a 90-percent reduction without relying on the other things that are usually enveloped with autonomous vehicles," says Jeffery Greenblatt, a staff scientist at Lawrence Berkeley National Laboratory in California, and lead author on the new study.
If autonomous taxis replaced conventional vehicles for just 10 percent of the average vehicle miles traveled in the U.S., emissions could drop by as much as 75 metric tons of carbon dioxide per year.
The researchers used projections of the greenhouse gas emissions for gasoline and electricity, courtesy of the United States Energy Information Administration. They compared those numbers to the relative efficiency of engine technologies to estimate the emission intensities per mile for internal combustion engine vehicles, hybrid cars, and battery-electric cars 15 years down the road.
Their simulations showed that if autonomous taxis replaced conventional vehicles for just 10 percent of the average vehicle miles traveled in the U.S., emissions could drop by as much as 75 metric tons of carbon dioxide per year, and the U.S. could save more than half a million barrels of oil per day.
A significant amount of those savings would come from the fact that the size of the taxis could be tailored to the number of passengers. Most trips in the U.S. are taken by single passengers, according to Greenblatt, and smaller, lighter vehicles would be more energy efficient. The team combined emissions projections with economic data and found that battery-powered electric taxis that last five years—comparable to the average lifespan of a taxi in New York City—would be the most cost-effective vehicle option in 2030.
Greenblatt’s critics question whether a battery-powered taxi—which typically put in 40 to 70 thousand miles a year—could really last five years. Cars in the U.S. last for just 150,000 miles on average, according to Greenblatt, but "diesel trucks regularly run for, like, a million miles before they’re retired.... There’s nothing intrinsic about an engine that prevents it from going past 150,000 miles." Battery technologies are constantly improving, and the right cooling system could keep a battery from degrading for 100,000 miles or so, Greenblatt says.
Google promises to have an autonomous car on the roads by 2017, but whether the robocars will become as ubiquitous as the human-controlled variety remains to be seen. "We are not predicting that these things will take over,” Greenblatt says. “We’re asking if [they do], what will be the environmental implications?"
The road ahead looks promising.
Quick Studies is an award-winning series that sheds light on new research and discoveries that change the way we look at the world.