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Speed limits are set for tractor trailer rigs. Modified based on numbers of accidents in the area (or complaints from locals).
I do get frustrated by highway speeds (seemingly) designed for the lowest-common-denominator of driving skill.Slow or fast, being predictable to other drivers and expecting their inattention, etc., seems to be key.
Yes, speed signs for curves are set for tractor/trailer rigs. Don't know about your other claim. When Carter was President it was to save oil consumption (55 mph) and that was when MC speedos in US had a 80 mph max. speedo. setting.