Why is the posted speed for a turn lower then the speed limit on most highway?
Answers
Often this is related to high sides vehicles, it vehicles with a high center of gravity (I'm looking at SUVs)
As you take tighter corners at speeds that may be safe it a straight line, the centrifugal force generated in the turn can result in vehicles with a high center if gravity to tip to the outside of the turn. Have you ever been in a car that's taken a turn quite fast, and you feel yourself being pulled to one side by the force. Like that only more so, as a lot more mass and momentum is involved.
Near my place of work there's a motorway on ramp that makes a tight (probably 120°) turn. This is as your probably accelerating from 30 mph to try and meet motorway speeds if 60–70 mph.
It is closed about 3–4 times a year to pick up a high sides heavy goods vehicle that has tipped over on the bend. It's a poor design.