Dangers of increased speed enforcement and lowered vehicle speeds.
It might be all very well suggesting that reducing traffic speed will cut accidents, but without considering the side effects of how this might be achieved it's a very dangerous game indeed. The evidence that reducing traffic speeds will reduce accidents is shaky too. 

Introducing this page:

This page lists side effects that might increase danger on the roads where drivers are forced to reduce speed by high levels of enforcement. Most research has ignored most of these possibilities. We are not aware of any research that has attempted to evaluate the degree of danger contributed by any of these effects.

How to use this page:
  • Column 1. Paragraph number. Click to see the comments page for the particular paragraph.
  • Column 2. Statement of effect.
  • Column 3. Brief description.
  • Column 4. email link to send us comments on the particular paragraph.
We will publish all suitable comments sent to us by email (including those we disagree with) on the comments pages.

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7.01 Drivers risk compensate and drive closer or more aggressively.  According to theories of risk homeostasis, we might well expect a driver at a slightly reduced speed to reset his personal level of risk back to “normal” by driving closer to other vehicles or by driving more aggressively. These effects might be tiny in the survey area, but might extend over a much wider area. This will probably be an important factor in accident displacement. mail
7.02 Drivers pay less attention at the new lower speed due to investing "just enough" attention for their personal level of safety.  This effect alone could eliminate the entire benefit of a planned speed reduction. Drivers set levels of safety for themselves, and one way of maintaining the same level of safety at a lower speed is to simply pay less attention. Inattention is frequently cited as a cause of up to 30% of all accidents, so increasing inattention is very risky. mail
7.03 Reduced individual driver responsibility in general and for speed in particular, leads to a reduced tendency for drivers to reduce speed when necessary.  The concern here is that drivers may become very gradually accustomed to relying on external advice for setting speed. On occasions they fail to reduce speed locally in the vicinity of road hazards and the frequency of this error is likely to increase. This effect may develop very gradually over months or years, and is more likely in less able drivers. In time this effect will tend increase the observed relationship between speed and accident rates in the manner of a self-fulfilling prophecy. mail
7.04 Less stimulation for drivers (lower work rates / lower information rates), leads to more sleepiness and poorer concentration.  It is very well known amongst skilled and advanced drivers that lower speeds demand lower attention, and that lower speeds promote lower attention. If we slow down vehicles over a wide area, particularly if we slow them significantly below the speeds that drivers are currently choosing, we risk increasing accidents due to inattention, poor concentration and sleepiness.

On long routes at lower speeds we would expect more drivers to fall asleep, partly because of increased journey times, but mainly due to lower levels of concentration.

7.05 Drivers' priorities are distorted. (i.e. speeds are set to legal limits rather than for safe driving reasons) Drivers avoid road hazards by observation, anticipation and planning. Speed should be adjusted to suit the circumstances. The more we tend to replace good driving practises by concentration on numerical speed the more dangerous drivers are likely to become. We obviously need our drivers to concentrate fully on setting safe speeds for the circumstances. mail
7.06 More distraction from enforcement. (i.e. drivers watch their speedos or watch out for cameras or police speed traps more than before, and watch the road less as a consequence) We depend on drivers paying proper attention to the road ahead for primary accident avoidance. It is obvious that high profile speed enforcement will have drivers making more frequent speedometer checks, looking for speed cameras and even looking for Policemen hiding in bushes. These activities must reduce their attention to the road ahead. mail
7.07 Traffic diverts to less safe roads due to enforcement on busy routes.  Obviously this will take place. We must evaluate the degree and the consequences. mail
7.08 The risk of accidents directly caused by enforcement.  We all know that on occasion drivers brake hard when they see an enforcement camera. Several accidents including one recent fatal are known to have resulted from this behaviour. The risk and responsibility aspects of the behaviour are down to the driver at the time, but the penalty for concern for licence should not be death. Also note that drivers travelling within the speed limit exhibit the behaviour for two reasons: a) If they check the speedo first, it may be too late to reduce speed and b) Despite the fact that they know what the speed limit is, they may not have the degree of certainty required to bet their driving licence on it, especially when faced with a snap decision. mail
7.09 The risk of injuries caused by enforcement hardware. Obviously if someone is injured in an accident where a vehicle strikes a fixed camera installation, neither the impact nor the injury could have occurred if the camera had not been there.  mail
7.10 Journeys take longer and cost more.  If the average speed of traffic is reduced there will be very significant effects in congestion and economic costs of transport. Each road user who is making an essential or important journey will take longer to complete it. Obviously these effects might be hard to quantify, but equally obviously if traffic is slowed there will be economic costs. In town vehicles will be less efficient and cause additional pollution. mail
7.11 Longer exposure to accident risk due to longer journey times.  Some accident risk on the roads is time based. Where journeys take longer, the time exposed to danger is increased. This effect must be quantified and allowed for. It is especially relevant for "fell asleep at the wheel" type accidents. mail
7.12 Joy riders, drunks, reckless and lawless drivers unaffected.  If we get to a point where we have figures to extrapolate road safety improvements from speed reductions, it must be remembered that not all road users will be affected by enforcement schemes.  mail
7.13 Poorer public / police relationship.  Road traffic policing has already had a clear effect on the public's perception of the police. Many law abiding people only come into contact with the police over road traffic issues. The blunt nature of the law, with eroded presumptions of innocence, eroded right to silence and absolute offences frequently leads to the Police being seen in a bad light. mail
7.14 Reduced incentive to train drivers better. The more we characterize drivers as “incompetents who must be regulated” the further we move away from the previous “individual responsibility” system of road safety that served us so well (providing excellent reductions in road casualties) until about 1990. The present course of speed reduction tends to lead us to neglect the basic sound idea of obtaining improved safety standards by training to improve drivers’ attitudes and road safety culture.  mail
7.15 More lawless drivers 

(e.g. false number plates, improper registration, no insurance, car cloning etc.)

It's obvious that we already have drivers who neglect or evade registration and other legal requirements in order to be safe from modern dumb speed enforcement. Once they have decided to behave outside the law we suggest that they may well behave in a more dangerous manner, and for example, might be much less likely to stop and render assistance after an accident. They are hit and run drivers in the making. mail
7.16 More safe drivers convicted with possible loss of job / home etc.  Whatever way you look at figures, it is clear that the vast majority of modern speeding offences are carried out by drivers who will never be involved in an excessive speed accident. One might reasonably infer that many cases of exceeding a speed limit take place in safe circumstances where no actual danger is caused. Applying the weight of law in these cases has serious consequences and sometimes results in loss of job. There probably already have been suicides triggered by the consequences of speeding convictions. mail
7.17 Reduced respect for law.  The vast majority of UK drivers regularly exceed the speed limit, and this includes the legislators, Police (without an emergency need) and court officials. When a citizen faces conviction for a speeding offence he knows full well that those responsible for convicting him are guilty of the same offence. The hypocritical application of the law brings it into serious disrepute and the ultimate consequences can only be guessed at. There are many ways that reduced respect for the law can bring new dangers to our roads. mail
7.18 new Drivers' general attitude to driving is worsened. One of the key factors that identifies a low risk driver is having a good attitude. A good attitude comes from taking responsibilities seriously and goes towards allowing safe margins for error. Drivers with a good attitude learn from their mistakes and don't take safety for granted. There's a significant risk that excessive speed enforcement will have a general negative effect on drivers' attitudes.  mail

Summary points.

Several recent reports (notably TRL511 and TRL421) purport to have found a relationship suggesting "for every 1 mph that traffic speeds are reduced, there will be a 5% reduction in accidents". The government appears to be planning traffic speed reductions based on the finding of these reports. However, there are very serious flaws in the approach. In particular:

  • Neither report established that the apparent link between accidents and speed was a causal one. So it's completely invalid to suggest that reducing speed will reduce accidents based on the findings of the reports.
  • Neither report properly considered any of the possible side effects listed on this page.
  • Both reports found that broadly "faster roads are safer", but went on with a system of sub classification of roads in attempt to prove that faster travelling was more risky on a given type of road. The system of sub classification is a fraught with problems, and despite considerable efforts, many anomalies remain unresolved.
Until these points are clarified it is dangerous and irresponsible to embark on a scheme of traffic speed reduction, since it is quite likely that doing so will increase accidents and injuries. Every item listed on this page should be researched, considered and quantified before embarking on such a scheme.

Visit our TRL421 page.

You can't measure safe driving in miles per hour

We have a strict editorial policy regarding factual content. If any fact anywhere on this web site can be shown to be incorrect we promise to remove it or correct it as soon as possible.
last updated 21st November 2002