When it's heavy enough to cover the pavement, snow blocks the view of lane lines that vehicle cameras use to find their way. Researchers so far haven't figured out a way around this. That's why much of the testing is done in warm-weather climates such as Arizona and California.
Heavy snow, rain, fog and sandstorms can obstruct the view of cameras. Light beams sent out by laser sensors can bounce off snowflakes and think they are obstacles. Radar can see through the weather, but it doesn't show the shape of an object needed for computers to figure out what it is.
"It's like losing part of your vision," says Raj Rajkumar, an electrical and computer engineering professor at Carnegie Mellon University.
Researchers are working on laser sensors that use a different light beam wavelength to see through snowflakes, said Greg McGuire, director of the MCity autonomous vehicle testing lab at the University of Michigan. Software also is being developed so vehicles can differentiate between real obstacles and snowflakes, rain, fog, and other conditions.
But many companies are still trying to master the difficult task of driving on a clear day with steady traction.
"Once we are able to have a system reliably perform in those, then we'll start working toward expanding to those more challenging conditions," said Noah Zych, Uber's head of system safety for self-driving cars.