I'm pretty confident Waze routes people not by posted speed limits, but by average driving habits and traffic speeds.

I do think Waze users are self-selecting. People that are willing to take "short cuts" choose Waze. I often ignore Waze's suggestions and have had people in the car with me comment on the extra turns and backroads.

There's a certain type of driver who prefers to mostly "tune out" and take the most direct route versus someone who wants to get there with the quickest ETA even if that means a more complex route.