"advisory" was ambiguous - I meant say, the lower speed limit is advisory - as in "45kmh an hour when wet".
I live in the middle of Sydney. This is an urban road. It is directly off a major highway in a suburban residential area.
It is a regular residential suburban street. No amount of "clever planning" will undo the natural topography of the region. It is a paved, well maintained road and that's the problem - people's judgement of what "feels right" depends on numerous factors they can't see and which don't matter.
They're in the middle of a recently resurfaced, asphalt road with a footpath down the side and what looks like trees and bush on side, and a cliff cut on the other. But it's relatively steep, winds a fair bit due to the climb, but also looks isolated when you're at the bottom because it runs through a state park area.
From street level you cannot tell how slippery it might be when wet (which people just plain suck at), how wet "wet" actually has to be (i.e. partially wet roads are more dangerous then when it's a hard downpour because the surface becomes slick), and unless you paid close attention to the area you can't know that there's no real protection along the side of the road (which shouldn't even be a factor: no one should be driving in a way where they depend on crash severity safety measures).
Observably, people's judgement of "feels right" sucks because as noted: there's been a fair few crashes basically caused by people taking corners too fast (which is to say, maybe they were speeding but that again is the point - they think they can safely go faster, and no, they actually can't and aren't good at judging that) - one of which was a car which very luckily ploughed into a very sturdy tree stump and didn't send it's occupants down the drop into the gulley.