Waymo's driverless cars ran out of charge, blocked San Francisco traffic, and one drove over a lit firework and caught fire over the Fourth of July.

Some of Waymo's driverless cars stalled during July 4th events in San Francisco. The company says the disruptions were caused by heavy traffic.

“Major traffic disruptions, a high volume of travelers, and unplanned road closures contributed to unexpected congestion,” Waymo said.