Edmund King, executive director of the RAC Foundation Driving on the M4 today I witnessed several cars passing a marked police car at 80mph.
They were all breaking the law, but the police took no action as the unwritten law suggests that drivers will not be stopped in fine weather conditions at a speed of 80mph.
Is 80mph too fast on the motorway? The safety lobby will automatically tell us that speed kills so any increase is dangerous.
Eighty miles per hour for an alert driver in a modern car on a clear day, travelling at a safe distance from the car in front on a well surfaced motorway is safe.
Fifty miles per hour for a tired driver in an old van on a rainy day tailgating the car in front on a potholed motorway is incredibly dangerous.
Some argue that if 80mph was the limit people would drive at 90mph. This is not necessarily so. The cars I witnessed driving at 80mph today did so because they knew they would not be stopped.
If the limit was increased to 80mph in fine weather with an understanding that people driving at over 85mph would be stopped then the majority of people would not drive at 90mph.
The problem with today's interpretation of speed limits is that if people know they will get away with breaking the motorway limit they assume they can break other speed limits with impunity.
We need a more flexible and intelligent approach to speed limits. But we also need actual facts on the contribution of speed in accidents.
We have variable speeds on the western section of the M25 so why not variable speeds from 20mph to 80mph on our roads depending on weather conditions or time of day? If motorists respect and understand speed limits, they will be more likely to comply with them.
Kevin Clinton, Royal Society for the Prevention of Accidents, head of road safety.
Drivers and riders who exceed the speed limit, or even those travelling under the speed limit when this is too fast for the conditions, cause or contribute to over 70,000 road crashes each year.
These 'accidents' kill over 1,000 people and seriously injure almost 13,000 others every year.
Not all speed related accidents are caused by speed alone.
Speed magnifies other errors, such as driving too close or driving when tired or distracted.
And driving too fast removes the safety margin.
So raising the motorway speed limit to 80mph would cause more accidents, more deaths and more injuries. It is nonsense to suggest that drivers would suddenly start behaving and obey the new limit.
And, it is unrealistic to expect a massive increase in traffic policing to enforce a new limit.
In fact, some chief constables are diverting their traffic police to other duties.
Allowing each individual driver to decide when the higher limit applies in fine weather and when it does not, on any particular motorway, at any given time, would be a recipe for confusion and disaster.
Variable limits on the M25 are effective in reducing traffic congestion, but they are properly signed and controlled, and vary within the 70mph limit, not above it.
Driving too fast kills and injures tens of thousands of people every year. Speeding is not a minor offence, but a serious, dangerous and anti-social activity in which the driver places his or her own convenience above the safety and well-being of other people.
Intelligent speed limiters that restrict the vehicle's speed to the limit on the road they are using are being developed. If drivers continue to prove themselves incapable of choosing to drive at safe speeds, the choice may ultimately be removed from them.
Driving too fast for the conditions causes or contributes to one third of road crashes, says the Royal Society for the Prevention of Accidents (RoSPA).
There are, on average over 70,000 speed related road accidents each year on Britain's roads.
Hit by a car at 40mph, nine out of 10 pedestrians will be killed. At 30mph, about half will survive. But at 20mph, the death rate is only 1 out of 10.
Some 66% of drivers exceed the 30mph limit in urban areas and 25% of drivers speed on 40mph roads.