Why We Need Speed Limits
Scepticism about current British speed limit policy does not mean that we should dispense with limits altogether
It may be thought that since I have created a website with details of speed camera locations, limit reductions and "great driving roads" where people might drive the odd mph or two above the speed limit, I'm not really in favour of speed limits. A properly trained driver should be able to judge an appropriate speed without the help of a number painted on a tin sign.
It is certainly true that a highly skilled driver can in many circumstances drive at speeds well above the speed limit with an acceptable degree of safety, and this is what the police have to learn to do to gain a Class 1 driving certificate. However, the roads are not a racetrack, they are a means of transportation, and while driving can be a very enjoyable experience, people should not drive at grossly illegal speeds either for fun or to get somewhere more quickly. Having said that, if no danger is caused to others, the police should exercise very considerable discretion before prosecuting people for speeding on deserted roads. Of course, if speed limits are reduced to an extent that most drivers feel to be unreasonable and unrealistic, then it is hardly surprising if they are routinely broken if people think they can get away with it.
In reality, not all drivers are trained to anything like that extent, and even though I strongly advocate a greater emphasis on driver training I recognise that it is unrealistic to expect that the majority of the driving population will ever reach a standard equivalent to an advanced test pass. A speed limit is always going to be an imperfect means of indicating the appropriate speed for a road, but it is far better than nothing, especially for less skilled drivers, and therefore speed limits do have an important role to play in road safety.
The purposes of speed limits are twofold:
Although the "Montana experiment" and the existence of derestricted autobahns in Germany are often cited as evidence of the limitations of speed limits, I don't believe that the total abolition of speed limits is right for UK roads, which are in general twisty and heavily used. Maybe ultimately we could consider a 90 mph motorway limit in the UK, but above that the economic benefit to a small number of drivers is outweighed by the risk posed to others, in particular by greatly increased speed differentials.
There is a well-established principle in setting speed limits known as the 85th percentile rule, meaning that the limit should be set at or around the speed below which 85% of drivers travel in free-flowing conditions. This recognises that most drivers, most of the time, behave sensibly, and only the irresponsible minority are placed outside the law. A limit set on this basis is likely to be broadly respected. If speed limits are set well below the prevailing speed on a section of road, in an attempt to modify driver behaviour, they can be counterproductive.
Experience has shown that a change in speed limit of 10 mph (either up or down) is likely to result in a change in actual speeds of no more than 2.5 mph. Of course it is then possible to change the appearance of the road to alter drivers' perceptions of it, but does that become an anti-car measure as much as a road safety one? If drivers were travelling before at a speed appropriate to the characteristics of the road, then what was the problem anyway?
Also see my Guide to Speed Limit Setting which sets out my views on how speed limits should be applied to UK roads, which are closer to official government policy than the approach of many local authorities