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I am in Vic and not NSW, but I think the principles are much the same. A classic happened many years ago on the Frankston Freeway, which covers a distance of something like 10km or so from Frankston to around Edithvale. It is divided almost the whole of its length, in some areas the division is more than 20 metres, with many trees. During peak times this freeway carries many thousands of cars in both directions.
Originally it used to be limited to 110, then one day a car load of young people slammed themselves into a tree along a stretch of the road at a speed well in excess of the limit...something of the order of 140+. The immediate reaction was to lower the limit to 90 for a number of years and then to put it back to 100. If the accident had occured near the posted limit, I could see some logic in the change, but it would not be reasonable to assume that had the limit been lower, that the car would have been travelling at a slower speed and therefore the speed limit in this case was nothing to do with the accident. Essentially, the accident was inevitable, regardless of any posted limit. In this case, the speed limit should never have been reduced in the first place. The reduction to 100 may be justified if they have made some logical decision about where 110 limited roads are situated and found this freeway did not fit this definition.
Basically, I favour 110 on divided roads far from large cities and 100 on what most people would call a "freeway". If there is an accident on a road, and the speed of the car is more than 20 above the posted limit, it is not logical to assume that the car would have been travelling any slower had the limit been set lower, and therefore the accident cannot be used to determine if a change needs to be made. If there are accidents on a road where the speed is just over the limit, or more so if it is under, then there is good reason to assume that a change of limit may have an effect and should be considered.
If people are going to speed excessively, changing the limits will not effect their behaviour and so you need to look at something else.
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