The arbitrary speed limits are often because many city planners still use the 80th percentile rule. Basically, they do a traffic study, then set the limit at what 80% of people are comfortable driving at. So that means 20% will naturally feel like they can go faster. And as they reach the 99th percentile, they’ll feel like they can go much faster.

The issue with this 80th percentile thing is that it has very little grounding in traffic safety or reality; Many roads are needlessly wide and give drivers an unrealistic sense of safety. They’ll feel like they can go 40 or 50MPH, when it’s really a street that is cutting through a neighborhood and is frequented by children playing, bike riders, etc…