1 Government stats show vehicles travelling at 80mph use 25% more fuel than those travelling at 70mph. If we use less fuel the UK would be economically better off.
2 Driving would be very much less stressful for those obeying the speed limit as they would not be constantly having to change lane to avoid drivers coming up behind. This could result in less stress-related illnesses and make savings for the NHS
3 in a emergency stop a car travelling faster than 70mph will hit an obstruction 100m away at a much greater speed than the increase in the travelling speed. E.g at 80mph it will still be travelling at 30mph whereas the car obeying the speed limit will be stationary. Fewer accidents, fewer casualties and less damage to property would result, obviously saving the NHS, the police and the insurance companies money.
4 Each driver travelling faster than the speed limit is breaking the law willingly. This will affect their attitude to law breaking and to the police in a negative way, which could have implications for their conduct in other spheres of life.
Motorways are very easy to police for speed limits. Cameras can be set up on bridges a known distance apart and simply record the registration number of every vehicle with the time. The time between the records of pairs of cameras will give an accurate reading of the speed of every single vehicle passing between them. As it is an average speed, rather than a snap shot as recorded by a speed gun, this is a fairer system for controlling limits.