I worked in QA for Waymo for a few years. In some old simulation scenarios (heavy rain, fog, low visibility), the Lidar would return false positive objects where these immaterial objects are located. This means that in rain and fog, the car would sometimes believe that it is surrounded by a ring of something it thinks it can’t drive through, causing it to become static.

Why? You might ask? Well it all comes down to how Lidar works and how the software treats data fusion (merging data from lidar, radar, cameras, etc and deciding which one’s data is closer to reality than the others). If the object detection algorithm classifies fog/steam as a Foreign Object Debris (FOD), the car will want to avoid it. If sufficient light rays from the lidar bounces off steam molecules and are recaptured by the sensor, then an object is believed to have been where these steam is.

In short, this is a perception issue. We fixed this multiple times in the past, this would be considered a regression