Part of the idea behind the 85th percentile speed idea talked about in the article is that drivers understand the dangers around them and slow down accordingly.

Part of the idea behind the 85th percentile speed idea talked about in the article is that drivers understand the dangers around them and slow down accordingly. If there are a lot of pedestrians around, most drivers will slow down a lot. Not all of them, but that's why you ignore that last 15%. The one thing the article doesn't mention though is a phenomena of traffic queuing at speed, which happens with average speed cameras. It supposedly increases the chances of collisions, because everyone is going at the same speed, not paying enough attention (because little is changing) and not maintaining enough stopping distance. I don't have citation for this unfortunately. Maybe someone else has a link to the study.