When adjusted for miles driven, Waymos were involved in police-reported hit-and-runs at 2.4 times the rate of human drivers in Maricopa County, the analysis found. The records were collected from the five cities in Waymo's Valley service area: Phoenix, Scottsdale, Chandler, Mesa and Tempe.

Waymo's own roadside assistance crews have noticed the pattern. "It's quite shocking how many people just take off," one specialist told a Phoenix police officer, according to body camera footage. In another incident, a specialist remarked, "I'm surprised at how many hit-and-runs we honestly get, and where it's a good amount of damage. Like, do you not realize there's cameras on these?"

Waymo reviewed the methodology and said its researchers "appreciated the rigor and thought" behind it. But the company argues it is impossible to determine whether its vehicles are actually involved in more hit-and-runs, or whether Waymo simply reports the incidents to police more consistently than the general public. In a statement, Waymo said the safety of riders and communities is its "absolute priority" and that across more than 90 million fully autonomous miles in Phoenix, its vehicles cut crashes involving serious injuries or worse by 90% compared with human drivers. The company said hit-and-runs are uncommon relative to the more than four million autonomous miles its vehicles drive each week across more than 15 U.S. cities, but did not say whether it tracks hit-and-runs internally or how many it has recorded.

Safety experts offered two possible explanations for why drivers leave. The first is confusion about how to exchange information with a vehicle that has no driver. "If there's no one there to trade insurance cards with, then what do you do?" said Missy Cummings, a professor at George Mason University. Phil Koopman, a professor emeritus at Carnegie Mellon University, noted that "ordinary drivers don't go through how-to-talk-to-a-robotaxi training." A 2022 Chandler police report illustrated the problem: after a minor crash in a 7-Eleven parking lot, the at-fault driver approached the Waymo with her driver's license and vehicle registration in hand, but left when she found no one to give them to.

Waymo said it invests heavily in educating drivers on how to exchange insurance information and that drivers do not need to wait at the scene. People involved in a crash can scan a QR code on the driver's side door, contact Waymo through its app, fill out a form at Waymo.com/notify, call 844-263-9885, email insurance@waymo.com, or speak with a roadside assistance representative if one is present.

The second theory is that Waymos may be prone to a type of crash that gives human drivers an incentive to flee. Cummings said her analysis of publicly available data shows Waymos get hit from behind twice as often as the average human driver, pointing to "phantom braking," when an autonomous vehicle stops suddenly for something that isn't there. "Everyone hears about hallucinations with generative AI. These are real hallucinations," she said. "The cars see something that isn't there. And the car's response is to do an emergency braking because, to them, it appears that something is there." Koopman added that while stopping may be the safe choice for the robotaxi, following drivers are not expecting a sudden brake.

Waymo disputes that sudden braking is a significant factor, saying rear-end crashes are the most common type for both human and autonomous drivers. The company said that over 270 million autonomous miles, Waymos were involved in 41% fewer rear-end crashes causing airbag deployment than human drivers in its cities. A peer-reviewed analysis by Waymo found its vehicles were fully stopped in 72% of rear-end crashes that deployed airbags and 20% of those causing injuries, and that its vehicles were not hard braking in any of the airbag crashes or in 75% of the injury crashes.

Arizona law requires drivers who hit an unattended vehicle to locate the owner or leave their contact information. Both Cummings and Koopman said Waymo should make that exchange easier. "If robotaxi companies are putting vehicles on public roads that interact with other people, it should be on the robotaxi companies to figure out how to make that interaction happen," Koopman said. Cummings called for more research. "We researchers and the government need to do more to try to figure out what is going on with this and how to mitigate it," she said.