Uber's self-driving cars are back on the road Thursday, nine months after a fatal accident in Arizona stalled development and pushed the company into lengthy reviews.

The company says it conducted a "top-to-bottom" audit of its safety policies. It previously vowed to improve operations before returning self-driving vehicles to the road. Uber pulled all testing in March, after one of the company's autonomous vehicles struck and killed a pedestrian.

A self-driving Uber SUV recognized a pedestrian crossing the street but failed to slow down for the designated back-up driver to manually brake in time. The National Transportation Safety Board investigated the accident and found gaps in Uber's systems.

"Over the past nine months, we've made safety core to everything we do," Eric Meyhofer, head of Uber Advanced Technologies, said in a statement. "We implemented recommendations from our review processes, spanning technical, operational and organizational improvements. This required a lot of introspection and took some time. Now we are ready to move forward."

The company is resuming road tests in Pittsburgh, with approval from the Pennsylvania Department of Transportation. The company is also resuming manual testing, with a human driver directing the vehicle, in San Francisco and Toronto.

"We've reviewed and improved our testing program to ensure that our vehicles are considerate and defensive drivers," Meyhofer said. "Before any vehicles are on public roads, they must pass a series of more than 70 scenarios without safety-related failures on our test track. We are confident we've met that bar as we reintroduce self-driving vehicles to Pittsburgh roadways today."