Back in the mid-50s I was an instrument man ---
---for the Trans-Canada Hwy & we had to go through all the rules/regs of roads, bridges,culverts, etc.
One thing I noted was that they specified that basically ALL bends must be set for trucks at 70mph (bar rare occasions like mtns, etc) which is 110KPH (the fastest one might possibly see on a few rare bits of the hwys in Cdn) & not the crawling speed of 80KPH which is 50mph!!!!!
Think about it -- in the days of drum brakes on cages they still felt 70 mph was a future speed limit.