REVENUE OR SAFETY ?

We are told that speeding cameras save lives because they will stop us from speeding. So why does it take a couple of weeks before we get the fine ? Is that because it is harder for us to recall the instance which usually means we won't dispute the charge. Wouldn't it make sense that we were stopped at the time of the offence so it will slow us down from the accident they claim we would of had. Or maybe our donation to the council is the perfect recourse to going faster than what they say is right because they know best.