Overwhelmed by dense fog in San Francisco early on Tuesday morning, five of Waymo’s fully driverless vehicles suddenly parked by the side of a residential street in what appeared to be a precautionary measure, the San Francisco Chronicle reported. Another of its cars apparently came to halt in the middle of the street, the news outlet said.

Other vehicles were unable to pass as “baffled motorists flashed headlights and tried to maneuver around the jam,” the Chronicle said.

The traffic problems persisted until the fog cleared and the autonomous cars were able to resume their journeys…

It said that after a “brief” stop, its cars continued on their way, adding: “We have software updates planned to improve our fog and parking performance to address such situations in the future.”