Amazon.com’s self-driving-car unit, Zoox, is getting a green light from California to test autonomous vehicles on public roads without a safety driver, becoming the fourth company to receive the state’s blessing for truly driverless testing.

The permit allows Zoox to operate two vehicles without a driver behind the wheel, a step up from approval it received four years ago that required the presence of a person to take control if needed, the state’s Department of Motor Vehicles said in a statement.

While some 60 companies have permission to conduct autonomous testing with a safety driver in California, Zoox joins Alphabet Inc. unit Waymo LLC, Nuro Inc. and AutoX Technologies in a small club of truly driverless testers in the state.

Zoox will be allowed to conduct tests on public roads within a designated area in San Mateo County near its headquarters in Foster City, California. The tests are allowed only under fair-weather conditions and on streets with a speed limit of 45 miles per hour.

Amazon agreed to buy Zoox in June to bolster the e-commerce giant’s ambitions in automated package delivery and ride-hailing.