SAN FRANCISCO: On July Fourth, more than 100,000 people packed San Francisco’s waterfront for a foggy fireworks show over the Golden Gate Bridge. When they tried to go home, Waymo got in the way. With crowds milling around, Waymo’s driverless cars blocked intersections and choked traffic. Some drivers were stuck behind the electric robo-taxis for more than three hours. Several Waymos had to be towed after their batteries died in the gridlock. At least two drove over exploding fireworks, with one catching fire. Rose Peterson was inside the other. She and her fiancee had ordered a Waymo to return to their hotel. When the car stopped at an intersection, a man ran into the road, set a firework on the street and lit it. The Waymo drove forward, rolling over the firework as it exploded beneath the car. “We were just in absolute shock,” said Peterson, 30, who works in insurance in Walnut Creek, California. The car smelled like it was on fire, she said, but the Waymo kept going until it stalled at another intersection. She and her fiancé then got out and walked the rest of the way to their hotel, turning what was supposed to be a five-minute drive into an hourlong journey. What Peterson experienced is what the driverless car industry calls an “edge case,” which are the unscripted situations that no one trained the robo-taxis to handle. The problem is that edge cases appear to be piling up as Waymo, the leading autonomous car service, rapidly expands. Owned by Google’s parent Alphabet, Waymo has more than quintupled the number of autonomous cars it has on the road to nearly 4,000 today, up from about 700 early last year. Its vehicles are now in 15 metro areas, having gone fully driverless in Denver, Las Vegas, San