This is absolutely not how they are designed. Maybe in theory, but in practice I’d say its way more than 15% of traffic speeding.
Traditionally it is how they’re determined, but it’s possible that this percentile goes up as roads get widened and the speed limit is never changed, or if the speed limit is lowered when there are concerns with fatalities.
Folks drive at what they feel is a safe speed for themselves. The posted speed limit doesn’t really seem to impact much, when the road is wide, the lanes are big, and there aren’t many turns or traffic calming elements, people will go fast because others are going fast.
That’s been my lived experience, and generally is supported by research that its road design more than anything that dictates speed.
