There can be some confusion when trying to figure out when you may get a fine for exceeding the speed limit, when there is evidence that others get away with it. The fact is, that even a 1 mile per hour speed above the permitted law for that stretch of road, is breaking the law and subject to a fine. But in the real world, it is extremely difficult to do as well the practical reasons to why poloce do not push for this.
One reason is that speedometers in our cars are no always 100% accurate and with no way for the average driver to check whether your stated 40mph is actually 39mph of even 41ph, it would be unfair to try and prosecute, not to mention the police would also have to test every car they stopped or photographed for speeding.
Another reason, is that drivers should be looking at the road constantly with only a glance now and again at the speed. So speeds can vary, especially if accelerating away from danger, so preventing someone from exceeding by a small amount could make driving more dangerous. Of course exceeding the limit for a long period of time, will be seen differently.
Safety Net In Place
The reality is that prosecuting is more likely to occur when speeds are above 10% of the limit plus 2 mph. This safety net gives sufficient flexibility to deal with inaccurate speedometers and normal driving conditions. If the limit is 50mph then in theory a fine will only be issued should you drive 57mph or more. But as was said earlier, this is not a cast iron guarantee and you would be breaking the law at 51mph.
Most offenses will be captured on speed cameras but more commonly average speed cameras are being used especially in areas such as road works etc. Some distances are quite long to measure average speeds, so it is very possible to drive way over the limit for some distance and way under for the rest of the distance and record an acceptable level. Of course the driver does not get a net gain, and this process allows for changing traffic patterns, but does not always deal with higher speeds in areas where workmen are working.
Earning Money Through Cameras
The public often have a negative view of speed cameras and rather than seeing them as a safety device to save lives, they can be seen as cash cows than earn the government millions of pounds each year in fine revenue. It is easy to measure how much money has been raised through camera but impossible to predict how many lives have been saved had traffic not slowed down. So the anti camera campaigns will always have data to share to prove their point. Most motoring agencies support cameras where there is a proven safety issue on certain stretches of road but are against them in areas where they feel they are their purely to generate income.
But the law is the law and to be adhered to, failing to do so may end up with a fine or worse a custodial sentence should the offence be serious enough.
More can be found at the Telegraph