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Speeding: An excellent law destroyed by dumb enforcement.
We examine the law against speeding and question current enforcement practice.


The offence of exceeding the speed limit was sensibly created to improve road safety and offer guidance to drivers. When it is applied with intelligence it can do exactly that, and indeed generally until about 1985 that's exactly what we had.

Much modern publicity suggests that perhaps 29.9 mph is safe and legal, while 30.1mph is dangerous and illegal. Many people accept the law at face value and condemn speeding drivers as dangerous. But thinking people all know that reality differs. Sometimes 40 mph in a 30 mph limit is a perfectly sensible speed. More frequently on other roads at other times 30 mph is dangerously fast.

Speed limits are good. They provide excellent information to drivers about hazard density, and they guide less experienced road users away from exceeding safe speeds by wild margins.

But speed limits are by their very nature arbitrary. In almost all circumstances the speed limit is not the best speed to drive at. As local conditions vary the optimum speed varies too.

Setting speed limits properly

Here is the news. Most drivers understand well what speeds are safe to use in what circumstances, and can be observed all over the country setting safe speeds both over and under the limit every day. Traffic engineers have long known this and have a rule called the "85th percentile rule". It's based on a frequency against speed plot of observed vehicle speeds, usually passing a fixed point. The 85th percentile rule tells us that in typical circumstances 85% of drivers are not exceeding safe thresholds. Setting a speed limit at the 85th percentile level is usually safe and correct.

But some responsibility for setting speed limits has been passed down to unskilled councillors, who set speed limits badly based on unclear thinking or even absurd prejudice.

So our major gripe number 1 is that speed limits should always be set by skilled traffic engineers with due regard for 85th percentile considerations. They should always be set to consistent national standards, so that similar roads anywhere in the UK will tend strongly to have similar limits.

There are presently far too many new 30 mph and 40 mph limits on roads that used to be National Speed Limit (NSL), where a responsible driver at a responsible speed may be prosecuted since the limit was set using absurd criteria. These roads are often policed by speed camera, which is simply evidence of the current ridiculous obsession with numerical speed.

Drivers are being expected to believe that what was safe yesterday at 60 mph is now dangerous and against the law at 45 mph. In most cases it simply isn't true, and everyone who thinks about it knows so. In this way, many important speed limits risk being brought into disrepute.

Read more about 85th percentiles: (overview) and (compendium of research)

new The competence and usefulness of speed limits

As we have discussed, speed limits form a useful part of a road safety strategy. But in recent years speed limits have been promoted way beyond their level of usefulness and competence.

Perhaps you have read about the "Peter principle" where staff who perform well are regulary promoted to positions of higher responsibility until they end up in a job where their level of competence is exceeded. Then the promotions stop, but people do not perform well in a situation which in often more demanding than their skill.

And so it is with speed limits and speed enforcement. Speed limits are properly a useful guide, and an easy weapon to use against those exceeding safe thresholds. But they have been erroneously promoted to much higher status.  There are all sorts of assumptions and claims about the safety benefits of slower traffic, very few of which have ever been delivered.

It's complete nonsense to suggest that 31mph is dangerous and 29mph is safe. It's even crazier to suggest that 31mph is dangerous today on a road where 60mph was permitted yesterday. There's so very much more to safe driving than that.

Many people - including police, scamera partnership staff and ordinary drivers have received the message that speed limits and speed enforcement are supreme (or at least of towering importance) in road safety. The truth is that they are of pretty minor significance, with a dozen or more other factors being far more important. 

How to use the speeding laws to make the roads safer

We propose getting rid of the dumb cameras and giving clear guidelines to the police to issue speeding tickets to drivers who are using excessive speed dangerously, carelessly, recklessly or aggressively. This might include ignoring lone cars at 110 mph on empty motorways and issuing tickets to drivers passing school gates at 3:45pm at 32 mph.

The conditions at the time of the offence make a far larger contribution to the degree of danger than the number on the sign. A motorway in fog and busy traffic might be extremely dangerous to drive at 69 mph, while the same motorway a few hours later when the fog and the traffic have both cleared might be perfectly safe at 100 mph.

The Police are frequently very aware of dangerous uses of speed, but current enforcement policies make no distinction whatsoever. Doubtless there are some traffic officers who do apply their intelligence and discretion, but this has become the exception rather than the rule.

There are dangerous cases of speeding out there. We should be using the law to address the problems. But prosecutions of safe drivers at safe speeds just cause misery and do not provide any social benefit. Worse than that, valuable speed limits, the law and the police are being brought into various kinds of disrepute. Virtually no one regards a driver with a modern speeding conviction as a dangerous individual to have on the road.

So we ought to change something. We need the speed limits to guide the inexperienced and to prosecute the dangerous and the incompetent. So we propose the following:

SafeSpeed Proposes:

Ask the police to enforce speed limits whenever and wherever danger is caused. Give them clear guidelines about judgements that they are expected to make at the time of the offence, with regard to all prevailing conditions, and the manner of driving of the target vehicle.

The main arguments:

When people first hear the proposal, they sometimes raise questions or objections as listed below.

What about the speed cameras?

They can't tell if an offence is dangerous or not, so they'll have to go. It's possible that there are some worthy exceptions, perhaps at accident black spots or places of temporary danger. Where they are justified the camera's job will be to slow drivers down, and that will work only if they are highly visible and well signed.

It's the law. Therefore it should be enforced.

Actually, the law should be enforced in the public interest.

The Police cannot (and should not) pursue all violations equally. They must set proper priorities. Murders and missing kids take higher priority than stolen paper clips. So why not set motoring offence priorities based on danger caused? It isn't supposed to be a numbers game.

It would lead to arguments in court, and only the lawyers would benefit.

No it wouldn't. We're not proposing changes in law. Speeding tickets would be prosecuted just as they are now. The difference would be that the police would not issue unnecessary tickets and would concentrate on causes of danger on the roads.

I wouldn't trust the police to make the judgement.

The alternative is trusting the judgement of an official hundreds of miles away who set the limit without regard for weather or traffic conditions. We'd rather trust the judgement of the Police at the scene. Generally the UK Police do an excellent job and are probably still the best in the world. UK Police driver training standards are very high, and the Police have a very good understanding of road dangers. If asked to enforce speed limits on the basis of danger caused we're confident that they could do an excellent job. There might be a few injustices, but only at a few percent of current levels.

With rigid enforcement of speed limits everyone knows where they stand.

Everyone knows that they are expected to drive safely. If a new or inexperienced drives doubts the safety of a speed then he would be extremely well advised to stick to the speed limit anyway. If an experienced driver knows that a certain speed is safe for the circumstances he would be very unlikely to be affected by misapplied policing. The standards which we should expect from drivers involve safe, sensible and responsible behaviour. Those that show those qualities should have nothing to fear, it's the others who should be worried. We should be promoting safe driving, not blind obedience to speed limits.

Your proposal is equivalent to scrapping fixed speed limits.

No it isn't. Anyone exceeding the speed limit without the certain knowledge that to do so is safe risks prosecution. Anyone that lacks the required experience to know that a particular speed is safe would therefore be very well advised to keep within the posted speed limit. Our proposal is entirely in line with the design intention of UK speed limits last time they were reviewed by the (then) TRRL in 1963. Enforcement practise followed this intention until fairly recently. 

It isn't rigidly enforced: they allow +10% +2mph in most cases.

Actually, allowing the for minor excesses over the limit does nothing to improve the situation. We could, for example, alter the motorway speed limit to 79 mph (=+10%, +2mph) and then allow zero tolerance. The effect is virtually the same as now. The circumstances of the offence are not considered, and we're asked slavishly to adhere to exactly the same numerical speed. The prosecution thresholds are the same. (i.e. exceed 79 mph and expect a ticket).

The changes we propose have a completely different effect. We give responsibility back to drivers, and ask them to use speed safely at all times.

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Created November 2002. Last update 7/03/2004