When most people ignore it. According to the Times,
The proportion of cars exceeding the 70mph limit was 57 per cent in 2003, up from 54 per cent in 2002.
I don’t know how this is measured, but it means that at least this number break the motorway speed limit at some time. On certain sections of road (for example the M3 between the M25 and London) in good traffic and weather conditions, one is hard pushed to find a single vehicle doing less than 80mph.
Some questions: what does it mean in a democracy when the majority ignore a law? Is the 70mph limit merely a pragmatic attempt to control the average speed, rather than a serious law intended to be obeyed? It certainly is not enforced.
I’m torn on the issue. I don’t like being threatened with the violence of law for something that is not wrong. On the other hand, I’d hate to think that all laws were rigourously enforced in the belief that they are perfect. From that perspective, the idea of a law that everyone just knows you can ignore as long as you don’t ignore it too much doesn’t sound so bad.
In an ideal world, of course, roads would be privately owned and their owners would set the rules. What I think would happen if this were the case, is that the rules would mostly standardise for reasons of practicality and the economics of liability. Insurance companies would more or less dictate speed limits to a level that minimised their costs. They would be pragmatic and likely set a lower limit than was enforced to control the average speed, understanding that it is human nature to bend the rules. In other words, pretty much what I think is happening in the real world with the motorway speed limit.
Things would get interesting if the speed “limits” set for purley pragmatic reasons were then strictly enforced. The Times article I linked to dates back to April 2005, and is what I found when investigating what the speed camera signs on the M4 were all about. The idea that motorway speed limits might actually be enforced is, frankly, a little shocking. Clearly no-one on the M4 could quite believe it because they were all happily speeding despite the signs. Thankfully, rumours that a system was in place to automatically record the average speed of all vehicles turned out to be just that. There was no sign of the mobile camera units mentioned by the Times, either. Perhaps they are not used because it was realised early on that they caused traffic to bunch up and increase the accident rate. Perhaps there are no real cameras and the signs are just another pragmatic attempt to play on human nature and get people to slow down and pay attention a bit more.
The problem is that roads are not privately owned but controlled by the state. It is probably only a matter of time before something like SPECS is implemented on motorways by power-crazed politicians because they, unlike insurance companies, are not bound by economics.
A final question: what would happen if motorway speed limits were either abolished, or raised to 100mph? Would everyone drive everywhere at 100mph or would they settle at between 80 and 95 like they do now? Given behaviour I’ve seen on rural roads which often have limits set higher than you can safely drive, I predict the latter.