Here's why self-driving cars can't handle bridges

Uber's driverless cars are hitting the streets of Pittsburgh, Penn. by the end of this month. And for anyone familiar with Pittsburgh, that means going over a ton of bridges.

Unfortunately, that's a bit of a problem for driverless cars, as Raffi Krikorian, Uber's engineering director, recently told Bloomberg.

"Bridges are really hard," Krikorian said. "And there are like 500 bridges in Pittsburgh."

During a recent test drive crossing the Allegheny River, Uber's driverless Volvo signaled to the driver on-board to take the wheel. It was only for a few seconds, but it shows that the driverless cars aren't ready to handle bridges without supervision.

That's because Uber has meticulously mapped roads so that the driverless car can compare what it's seeing with what is supposed to be there, helping it avoid objects and pedestrians.

Because bridges don't have many environmental cues like surrounding buildings, it's hard for the Uber car to figure out where it is, Krikorian told Bloomberg. GPS helps the car position itself, but not to the accuracy Uber wants.

John Dolan, principle systems scientist at Carnegie Mellon's Robotics Institute, told Business Insider that bridges can pose other problems for driverless cars, as well.

"You have a lot of infrastructure on the bridge above the level of the car that we as humans take into account," he said. "But when you sense those things with a sensor that doesn't have the domain knowledge that we do ... you could imagine that the girders coming up from the side of the bridge and that kind of thing would be disturbing or possibly confusing."

Bridges are one of a few difficult scenarios driverless cars can't handle, meaning we'll still be seeing a human behind the wheel for the foreseeable future.