The toughest challenge for self-driving cars? Human drivers

FILE - In this Wednesday, Sept. 14, 2016, file photo, a self-driving Uber car stops at a red light on Liberty Avenue through the Bloomfield neighborhood of Pittsburgh. In just a few years, well-mannered self-driving robotaxis will share the roads with reckless, law-breaking human drivers. The prospect is causing migraines for the people developing the robocars and is slowing their development. But experts say eventually the cars will coexist with human drivers on real roads. (AP Photo/Gene J. Puskar, File)

DETROIT —  In just a few years, well-mannered self-driving robotaxis will share the roads with reckless, law-breaking human drivers. The prospect is causing migraines for the people developing the robotaxis.

A self-driving car would be programmed to drive at the speed limit. Humans routinely exceed it by 10 to 15 mph  just try entering the New Jersey Turnpike at normal speed. Self-driving cars wouldn’t dare cross a double yellow line; humans do it all the time. And then there are those odd local traffic customs to which humans quickly adapt.

In Los Angeles and other places, for instance, there’s the “California Stop,” where drivers roll through stop signs if no traffic is crossing. In Southwestern Pennsylvania, courteous drivers practice the “Pittsburgh Left,” where it’s customary to let one oncoming car turn left in front of them when a traffic light turns green. The same thing happens in Boston. During rush hours near Ann Arbor, Michigan, drivers regularly cross a double-yellow line to queue up for a left-turn onto a freeway.

“There’s an endless list of these cases where we as humans know the context, we know when to bend the rules and when to break the rules,” says Raj Rajkumar, a computer engineering professor at Carnegie Mellon University who leads the school’s autonomous car research.

Although autonomous cars are likely to carry passengers or cargo in limited areas during the next three to five years, experts say it will take many years before robotaxis can coexist with human-piloted vehicles on most side streets, boulevards and freeways. That’s because programmers have to figure out human behavior and local traffic idiosyncrasies. And teaching a car to use that knowledge will require massive amounts of data and big computing power that is prohibitively expensive at the moment.

“Driverless cars are very rule-based, and they don’t understand social graces,” says Missy Cummings, director of Duke University’s Humans and Autonomy Lab.

Driving customs and road conditions are dramatically different across the globe, with narrow, congested lanes in European cities, and anarchy in Beijing’s giant traffic jams. In India’s capital, New Delhi, luxury cars share poorly marked and congested lanes with bicycles, scooters, trucks, and even an occasional cow or elephant.

Then there is the problem of aggressive humans who make dangerous moves such as cutting cars off on freeways or turning left in front of oncoming traffic.

In India, for example, even when lanes are marked, drivers swing from lane to lane without hesitation.

Already there have been isolated cases of human drivers pulling into the path of cars such as Teslas, knowing they will stop because they’re equipped with automatic emergency braking.