Even worse, there are school signs that say 15mph when children are present. Kids could be behind cars, bushes, people, etc. That’s a very hard task to deal with.
Humans are flawed but human drivers are way safer than the human detractors would have you think.
Not every human driver has the same risk, but every self driving car will. (Or it will be based on which car you are in rather than how safe you are.) In other words, relatively safe human drivers could actually see their risk levels go up in a self driving car, even it if it statistically safer than all human drivers.
Not really though, you could just assume there are kids and do 15mph. Not everything is a hard to solve machine learning challenge.