The National Highway Traffic Safety Administration (NHTSA) said on Tuesday that Waymo is recalling 3,791 robotaxis in the United States over a self-driving software defect that could cause the vehicles to drive onto flooded roads. The recall covers certain fifth- and sixth-generation Automated Driving Systems (ADS) deployed across the Alphabet subsidiary’s commercial fleet. According to the NHTSA, the software may fail to adequately detect or respond to flooded road conditions — creating a potential safety risk for passengers and other road users. Waymo is currently developing a full software remedy, the agency said. As an interim measure, the Alphabet subsidiary has tightened its operational parameters by increasing weather-related constraints and updating its map data to restrict vehicle access to roads that may be affected by flooding. Waymos on Flooded Roads According to the NHTSA recall notice, on April 20 an unoccupied Waymo vehicle “encountered an untraversable flooded section of a roadway” with a 40 mph speed limit. The vehicle detected the floodwater but continued forward at reduced speed rather than stopping or rerouting. Following the incident, Waymo implemented additional operational restrictions “at times and in locations where there is an elevated risk of encountering a flooded, higher-speed roadway,” the filing said. Four days later, Waymo‘s Safety Board reviewed the issue and decided to conduct a recall. The April 20 incident does not appear to be an isolated case. Users on social media have shared images and videos of Waymo robotaxis entering flooded areas or becoming trapped in standing water. The incidents include at least one situation in Phoenix, where the vehicle entered a flooded road, ignoring a sign that said “do not enter when flooded,” and then went into reverse. Recall Covers Nearly Entire Fleet While Waymo does not routinely disclose its exact fleet size, available data suggests the recall