Or is there a deliberate trap-setting motive, whereby the police know they will get a good return from booking 'speeders' in these zones?

'Speeders' who are doing a perfectly safe speed that is lower than the normal speed limit for the particular section of road, that is!

These blokes are the growth employment industry. I saw one yesterday and he had to be 80 and a poor walker. He was putting cones out so the council could cut the grass on the median strip.
There are six separate road safety sign companies here in Cairns. God knows how many around the country but cockroaches are more rarer than these blokes.