It appears precipitation is enough to require the human back-up driver to take control in a Waymo self-driving car.

According to a report from The Verge, rain was one of a few factors that prevented Waymo's self-driving car from carrying out its autonomous duties. The insight came from one of just a few people who Waymo has cleared to be a part of the Waymo One ride-hailing service. The company launched the service late last year in Arizona with a fleet of Chrysler Pacifica minivans. However, according to the report, the service sometimes falls short of promises the company made. 

The rider, Shawn Metz, provided a hefty dose of first-hand knowledge on the self-driving cars and described an instance in which rain was enough to stop the self-driving system. He recalled a time when he requested a Waymo One for a ride during a rainstorm. When the car arrived, the human driver was driving the car manually. Metz didn't provide an explanation as to why or how severe the rain was, but clearly the human was smarter during the inclement weather.

Metz also mentioned a time when he requested a Waymo One for a ride to the local Costco. The self-driving van wasn't able to maneuver through the crowded parking lot during the holiday season. Metz said the vehicle basically got stuck outside of the entrance as it tried to find a way through the numerous pedestrians. Eventually, the vehicle timed out and the backup driver had to call the support center for a re-route. 

Metz also noted that the fees for Waymo One are comparable to those for Uber.

Waymo One only operates in a small area of Arizona (about 100 square miles around Phoenix) and remains closed to the general public. The service only caters to a small group of people, but it's unclear how many. Metz wasn't sure how many are part of the program, but Waymo has said it plans to add "hundreds" of new riders from the First Riders program, which included about 400 people that were screened and accepted for beta testing, in the near future.