Waymo robotaxis stalled in traffic — and drove over exploding fireworks — on July 4th

As Waymo's fleet nears 4,000 cars, a chaotic San Francisco fireworks night shows how rare "edge cases" scale up with growth.

What happened: On July 4th, more than 100,000 people crowded San Francisco's waterfront for fireworks. As the crowd dispersed, Waymo robotaxis blocked intersections and jammed traffic, leaving some riders stuck for over three hours. At least two cars drove directly over exploding fireworks; one caught fire and later stalled at the next intersection. Passenger Rose Peterson said she smelled smoke inside the car and was in "absolute shock" as it happened around her.

Why it matters: Waymo calls incidents like this "edge cases" — situations too unusual to have been explicitly programmed for. But federal regulators pushed back hard in a July letter, arguing that emergency scenes, crowds, and chaotic street conditions are common enough that they shouldn't be waved off as rare exceptions. As Waymo's fleet has grown fivefold to nearly 4,000 cars across 15 metro areas, the sheer number of trips means low-probability failures now happen with real regularity somewhere in the network.

How it works, plainly: Self-driving systems are trained mostly on ordinary traffic patterns. Fireworks exploding at ground level, dense post-event crowds, and emergency vehicles all fall outside the most common training scenarios, so the car's software can misjudge what's happening or freeze up entirely. That's what "edge case" means in practice: not a bug exactly, but a gap between what the system has seen before and what the real world occasionally throws at it.

The rollout: Waymo maintains its cars still cause 94% fewer serious-injury crashes than human-driven vehicles, a statistic drawn from its much larger overall safety record. But regulators' letter signals growing federal scrutiny of how Waymo classifies and discloses these incidents as it keeps expanding into new cities. Expect more public pressure on the company to explain how it handles crowds, emergencies, and unusual street conditions before adding thousands more cars to its fleet.