They were all over the Netherlands when I lived there in 1995-1996. Of course if you were local you quickly learned where they were so they only trapped people from out of the area or new to the area.
Cars GPS's usually have a feature to warn you when you are on a road with speed cameras. Our Garmin, bought in the USA but with European maps also has this feature but when we were in the UK for many months in 2011 I eventually turned it off because there were always signs everywhere there were cameras and the warning beeps became annoying and not useful. It was the same last year in Australia where we did a lot of driving and used a GPS borrowed from my brother.
In 2013 we drove in France and were careful to note a couple of changes that year to french driving laws which were that you had to have 2 breathalyzer kits in the car and that the GPS speeding camera feature had to be turned off. This confirmed to many that in France the speeding cameras were mainly for revenue and safety came second.