Ride-hailing giant Uber Technologies said on Tuesday that it is expanding its self-driving pilot program to a city that isn't the easiest for human drivers to navigate -- San Francisco.

What Uber said

Uber said that starting on Wednesday, Dec. 14, "riders who request an uberX in San Francisco will be matched with a Self-Driving Uber if one is available."

As with the company's ongoing pilot program in Pittsburgh, which began about three months ago, Uber has added several company-owned self-driving vehicles to its regular ride-hailing fleet. 

The vehicles are Volvo XC90 SUVs equipped with the latest version of Uber's prototype self-driving system. As in Pittsburgh, the self-driving Ubers in San Francisco will have two Uber engineers on board, one of whom will be required to keep his or her hands on the steering wheel at all times. 

A Volvo XC90 equipped with Uber's prototype self-driving system in San Francisco. Several of the Volvos joined the company's ride-hailing fleet in the city this week. Image source: Uber Technologies. 

"Expanding our self-driving pilot allows us to continue to improve our technology through real-world operations," the company said in a statement. "With its challenging roads and often varied weather, Pittsburgh provided a wide array of experiences. San Francisco comes with its own nuances including more bikes on the road, high traffic density and narrow lanes.

"The promise of self-driving is core to our mission of reliable transportation, everywhere for everyone. As demand for ridesharing continues to skyrocket, the future of transportation will be a mix of human drivers and self-driving cars."

Why Uber's self-driving tests are limited to two cities (for now)

Self-driving systems like the one being developed by Uber require very precise 3D maps of the areas they travel in. Uber spent several months operating a self-driving test car in Pittsburgh before it launched its pilot service; that test car was equipped with a system to build and refine Uber's map of the city. It's likely that the vehicles in the pilot program have helped to refine the map since they began operations. 

As Uber said in its statement, San Francisco presents some new challenges for Uber's system. The company has presumably created a "good enough" map of the city to begin these larger operations. 

Why Uber is using Volvos in this pilot program

Uber's original test car in Pittsburgh was a Ford (NYSE:F) Fusion. Fusions have been chosen by several different companies as test vehicles for assisted-driving systems, business that Ford has sought out by offering technical assistance with integration.

But more recently, Uber announced a deal with Volvo. The Swedish automaker, now owned by China's Geely Automobile Holdings (OTC:GELYF), agreed to provide the ride-hailing giant with 100 XC90s that had been modified to accept Uber's self-driving system by the end of 2016.

Some of those vehicles have already been delivered and added to Uber's Pittsburgh fleet. Apparently, the next batch is now operating in San Francisco.

Why self-driving technology is critical to Uber

Uber is developing its own self-driving technology as part of a long-term plan to replace its human drivers, in hopes of lowering the cost (and improving the safety) of its service. 

About two years ago, Uber CEO Travis Kalanick recruited a group of robotics experts from Carnegie Mellon University to develop a system that could be retrofitted to existing vehicles. Uber has since purchased a start-up called Otto, which was working on self-driving tractor-trailer trucks, and plans to run an automated trucking service in addition to its core ride-hailing business. 

The important thing to remember is that Uber isn't just dabbling in this field. Self-driving technology is a critical part of Uber's long-term business plan. 

What's next for Uber's self-driving program

Presumably, the company will roll out pilot programs in more cities over time, as it continues to refine and polish its self-driving system. But it will be interesting to watch the company's effort unfold in tech-savvy San Francisco: Uber's customers in the city may turn out to be an exceptionally informed and critical audience. 

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