The problem is that speed restrictions are ignored by most people. It's so bad that even if someone wants to drive slowly, it becomes unsafe to do so. This means that people are often driving at 90kph+ in a 30, 50, 60 and so on, so when there's suddenly a sign saying they need to slow down, it is ignore like all the others, at least by people who are unfamiliar with the area.
It's very likely that the reason "danger curve" is so dangerous, is that people travel round it way too fast.