SAN FRANCISCO — Uber said its autonomous vehicles returned to public roads on Thursday, nine months after one of its self-driving cars killed a pedestrian in Tempe, Arizona.

But Uber’s driverless vehicles are operating at drastically reduced speeds and in less challenging environments than before, the company said, as it eases back into testing and tries to ensure people’s safety.

“We will continue to prioritize safety and proactively communicate our progress until we’ve built a self-driving system that lives up to the promise of making transportation safer and more affordable for everyone,” Eric Meyhofer, who leads Uber’s autonomous unit, said.

Uber said it received permission on Monday from the Pennsylvania Department of Transportation to test its driverless cars again on public roads in Pittsburgh. The company said it planned to put fewer than five cars on the road Thursday, and slowly add to that over time.

Each car will have two drivers inside, ready to take over in case something goes wrong, Uber said. The operators will work in four-hour shifts, down from eight to 10 hours. No passengers will ride in the cars.

The autonomous cars will go no faster than 25 mph down from as fast as 55 mph before. They will operate on a limited loop in Pittsburgh’s Strip District, a bustling commercial area, Uber said.

Uber said it was putting autonomous vehicles back on the streets in Toronto and San Francisco, although those cars will operate in manual mode, which means a human driver will control the vehicles’ movements rather than software.

The company had grounded its fleet of autonomous vehicles in March after one of its cars — with an emergency backup driver behind the wheel — struck and killed Elaine Herzberg, 49, on a street in Tempe.