> Traffic engineers study roadways and recommend speed limits based on safety and human behavior.
This isn't really true: what usually happens is that they either go with a default or they do a study and set the limits at the 85th percentile. For a separated highway, that works fairly well but it often has bad results for mixed spaces: the people commuting through a neighborhood, for example, are trying to go as fast as possible but the people who live there are more concerned about safety, the impacts of those decisions on how they use their space (I grew up hearing that “nobody walks in California” which really meant “nobody wants to walk 3 miles further to use the few signaled crosswalks”), etc. A big problem here are the outliers: most of the risk comes from the top of the speed distribution — even if half of the drivers scrupulously follow the speed limit, the speeders are the ones who will influence people's safety perception of the road.
This isn't really true: what usually happens is that they either go with a default or they do a study and set the limits at the 85th percentile. For a separated highway, that works fairly well but it often has bad results for mixed spaces: the people commuting through a neighborhood, for example, are trying to go as fast as possible but the people who live there are more concerned about safety, the impacts of those decisions on how they use their space (I grew up hearing that “nobody walks in California” which really meant “nobody wants to walk 3 miles further to use the few signaled crosswalks”), etc. A big problem here are the outliers: most of the risk comes from the top of the speed distribution — even if half of the drivers scrupulously follow the speed limit, the speeders are the ones who will influence people's safety perception of the road.