Unfortunately cameras do not prevent incidents as they are there to make money! What is needed is more traffic officers on the ground patrolling the streets. That is a far better deterrent than any safety speed camera. Many people know where the sped safety cameras are located and slow down for that area and then speed up once out of the zone. Also most Satnavs have a list of speed safety cameras anyway!
Ask yourself who is breaking the law?
If all drivers could be trusted to keep to the speed limits, we wouldn't need speed cameras. The cameras would not be cash generators if drivers took their responsibilities to uphold the law seriously. It's only those who choose to speed who complain about the cameras.
Whilst I agree that some camera's have earned a reputation as a cash generator, and how many drivers slow down for a known camera location then deliberately speed up above the limit after passing it, then complain about being them as if it's their rite to be able to speed and break the law.
But if by being present (in use or not) it has prevented a serious incident, then it has done its job and probably paid for itself.
If you don't like a particular piece of legislation, it does not give you a right to flagrantly ignore it and break it (e.g. speeding). You have the right to challenge it to try to get it changed through prescribed procedures, but as long as it's been lawfully instigated, it applies untill it's been changed. However I seriously doubt any individual could succeed to get a permanent speed limit raised or removed.