It was funny.. as I was driving along the other day, thinking much the same thing "so, well what makes this speed more dangerous that another".. In that, I travel 100km/hr it's legal, but another 5-10km/hr over its not whether it's wet, dry, or foggy or I'm half asleep, yapping away to the person in the passenger seat, etc. Likewise, what is "safe" in one car, would not be so safe in another. It's such a human thing to stick an arbitary number on something that is never entirely quantifiable.
I really think that is why the victorian model is so flawed.. 3km/hr.. come on! it's barely readable on the speedo, never mind trying to drive at it through a whole city with fixed non-descretionary cameras everywhere.
That said, I don't know how else you stop people going whatever speed they like, so some arbitary figures have to be chosen at some point. If it was like "150" on the highways, the nature of many would be to go 150 at the minimum, and then above and beyond that for some.
Another thing to lookout for is the usual propoganda yada during the holiday "carnage" periods.. Where they say "8 people have been killed on the roads this long weekend". But when you narrow it down, to say, last year x people were killed and divide by that by the number of days, more often than not this "carnage" is no worse than statistically average for any other 3 days of the year. Check into it next time you see it mentioned - it might make you think.