PITTSBURGH — In the world of autonomous vehicles, Pittsburgh and Silicon Valley are bustling hubs of development and testing. But ask those involved in self-driving vehicles when we might actually see them carrying passengers in every city, and you'll get an almost universal answer: Not anytime soon.

An optimistic assessment is 10 years. Many others say decades as researchers try to conquer a number of obstacles. The vehicles themselves will debut in limited, well-mapped areas within cities and spread outward.

The fatal crash in Arizona involving an Uber autonomous vehicle in March slowed progress, largely because it hurt the public's perception of the safety of vehicles.

Companies slowed research to be more careful. Google's Waymo, for instance, decided not to launch a fully autonomous ride-hailing service in the Phoenix area and will rely on human backup drivers to ferry passengers, at least for now.

Here are the problems that researchers must overcome to start giving rides without humans behind the wheel:

SNOW AND WEATHER: 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.

PAVEMENT LINES AND CURBS: Across the globe, roadway marking lines are different, or they may not even exist. Lane lines aren't standardized, so vehicles have to learn how to drive differently in each city. Sometimes there aren't any curbs to help vehicles judge lane width.

DEALING WITH HUMAN DRIVERS: For many years, autonomous vehicles will have to deal with humans who don't always play by the rules. They double-park or walk in front of cars. Recently in Pittsburgh, an Argo backup driver had to take over when his car stopped during a right turn, blocking an intersection when it couldn't immediately decide whether to go around a double-parked delivery truck.

"Even if the car might eventually figure something out, it's shared space, and it's socially unacceptable" to block traffic, Rander said.

LEFT TURNS: Deciding when to turn left in front of oncoming traffic without a green arrow is one of the more difficult tasks for human drivers and one that causes many crashes. Autonomous vehicles have the same trouble.

Waymo CEO John Krafcik said in a recent interview that his company's vehicles are still encountering occasional problems at intersections.

CONSUMER ACCEPTANCE: The fatal Uber crash near Phoenix last year did more than push the pause button on testing. It also rattled consumers who someday will be asked to ride in self-driving vehicles.

Surveys taken after the Uber crash showed that drivers are reluctant to give up control to a computer. One by AAA found that 73 percent of American drivers would be too fearful to ride in a fully self-driving vehicle. That's up from 63 percent in late 2017.

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