Select Committee on Transport, Local Government and the Regions Appendices to the Minutes of Evidence

Memorandum by the Transport Research Laboratory (RTS 27)


  The toll of death and serious injury resulting from too much speed, and the effects on the quality of life, are major issues of our time. Despite a very considerable body of evidence showing the effect of speed on increased accident frequency and severity, and the effects on communities, the problem remains endemic. Our intention in this memorandum is to summarise in outline the evidence from national and international sources and to submit our views on how the problems associated with speed should be tackled.


  When a 30mph speed limit was first introduced in British towns in 1935, deaths fell by 15 per cent (Finch et al, 1994). Between the 1950s and 1980s and onwards, many countries introduced new speed limits, or lowered existing ones, with the almost universal result that if they were enforced, speeds and accidents both reduced (eg Salusjarvi, 1988; DSIR, 1963; Engel & Thompson, 1988). TRL published results of a meta-analysis of these studies (Finch et al, 1994), concluding from the wealth of underlying data that, on average, each 1 mph reduction in mean traffic speed is associated with a 5 per cent reduction in accidents. In the US, increases in the national speed limit have been shown clearly to be associated with increases in fatalities (eg Garber & Graham, 1990).

  A number of important studies (see Taylor et al 2000 and Kimber 2001) have since addressed the relationship between speed and accidents directly, as follows:

    —  TRL studies of individual drivers have addressed the question of how drivers' speed choices affect their likelihood of being involved in an accident (Taylor et al 2000; Quimby et al, 1999a; Maycock et al, 1998). The accident records of more than 10,000 drivers were related statistically to their observed speeding behaviour, with the clear result that accident risk rises the faster a driver travels, in relation to those around him. At a speed of 25 per cent above the average speed, the risk of accident involvement rises more than 500 per cent. So the greatest reduction in casualties will come from reducing the speeds of the fastest drivers.

    —  TRL road-based studies addressed the question of how the speed on a given road affects accidents occurring there (Taylor et al, 2000). Several hundred thousand observations of vehicle speed on more than a hundred roads of different types were related statistically to the numbers of accidents occurring on those roads. Again, the major impact that speed has on accidents is clear: (i) the faster traffic moves on average on a given type of road, the more accidents there are. Injury accidents rise rapidly with average speed: approximately in proportion to its square—if the spread of speeds (the ratio of the standard deviation to average speed) remain constant. (ii) the bigger the spread of speeds around the average speed, the more accidents there are. The frequency of accidents rises rapidly—indeed exponentially—as the spread of speeds increases, for a given average speed. (iii) these effects of speed on accidents depend on the road type and situation.

    —  Many "before and after" studies of measures to slow traffic and improve safety have been reported. These measures include: traffic calming schemes in 20mph zones (Webster & Mackie, 1996)—where injury accidents more than halved; traffic calming in villages (Wheeler & Taylor, 2000)—where injury accidents reduced by more than 20 per cent; speed cameras (DTLR, 2001a)—where injury accidents reduced by a third; village speed limits (Suffolk County Council, 1999)—where injury accidents reduced by 20 per cent. This evidence for a link between reduced speeds and reduced accicdents is very strong in itself.

    —  Australian work examined speeds of accident-involved cars and found them to be higher than speeds of other cars, under the same conditions (Kloeden et al, 1997).

  Together these studies provide robust and compelling evidence of the nature and magnitude of the effect of speed on accidents. They relate to real traffic on real (largely UK) roads. They are large-scale studies involving rigorous statistical analysis. Very importantly, they take account (Taylor et al, 2000) of the complex interaction between the relevant variables, an issue which, if ignored, can lead to misleading conclusions.


  In the "before and after" studies above, the more severe the accidents the greater the reduction in their frequency. Swedish work (Andersson and Nilsson, 1997) also shows that whilst injury accidents are proportional to the square of average speed, accidents involving fatal/serious injury are proportional to the cube of the average speed and fatal accidents are proportional to the fourth power of the average speed. Other studies show increased probability of injury with impact severity (Hobbs & Mills, 1984). Higher impact speed means greater pedestrian injury severity (DETR, 2000).


  Illegal speed is speed in excess of the limit; inappropriate speed is speed that is too fast for the prevailing conditions (and may include illegal speed). Interpretations of these phrases vary and care needs to be taken in what is meant. Research relating to the distinct (ie separable) effects of illegal and inappropriate speed in accidents is not so extensive, but includes:

    —  The identification of a relationship between accident frequency and the proportion of vehicles exceeding the speed limit (Taylor et al, 2000). This shows that: firstly, the higher the proportion of drivers speeding, the more accidents. If this proportion of speeders doubles, accidents go up by 10 per cent. Secondly, the higher the average speed of those drivers who speed, the more accidents:— the frequency of accidents rises by 19 per cent if the average speed of the speeders increases by one mph. Again the potential to reduce casualties by reducing the speed of the fastest drivers is clear.

    —  "Contributory factors" data based on police records collected at accident scenes also provide some evidence about the role of inappropriate speed. These enable the proportion of accidents to be estimated in which inappropriate speed was deemed to be a factor (Broughton et al, 1998). The concept relies on the subjective judgement of a police officer attending the accident scene after the event, of the "appropriate speed" in a given situation, and of its connection with other, related factors. Whilst it is clear from these studies that inappropriate speed is a factor in a substantial proportion of accidents, the exact magnitude of this proportion is difficult to quantify. In contrast, the objective evidence relating directly to the speed-accident relationship (above) is demonstrably more relevant to: (i) quantifying the influence of speed in different situations, (ii) gaining a real understanding of the issues, and (iii) addressing the range of very different problems that exist.


  Most of the now extensive evidence relates to illegal speed. The AA Foundation (2000) suggests that this is what most people think of as being inappropriate. Quimby et al (1999a) found in a study of more than 5,000 drivers that site characteristics have by far the biggest influence on drivers' speed choice. Speed choice is further influenced by personal characteristics, which include demographic and psychological characteristics (Webster and Wells, 2000). In the former category, mileage travelled has been identified as a key factor—higher mileage drivers tending to be faster drivers (eg Quimby et al, 1999). The influence of psychological characteristics is well-documented (eg, AA Foundation, 2000; Corbett, 1999; Stradling, 1999; Quimby et al, 1999a,b). Non-UK sources are considered less relevant because of the differences in culture, legislation and enforcement between countries.

  Attitudes to speeding need to be changed (eg see AA Foundation, 2000) and road environments need to better explain the appropriate speed in different situations (eg DETR, 2000). A greater understanding is required of how to encourage people to understand/accept the need to reduce speed.


  There is a body of research considering factors that influence pedestrian casualty levels, and solutions (eg, Taylor et al, 1996; Summersgill et al, 1996 and others who have identified the determinants of pedestrian accidents on road links and at junctions of different types, and AA Foundation, 1994, which relates risk to road/housing type). Speed is not directly considered in these studies, but Taylor et al (2000) show that both pedestrian and vehicle accidents are reduced with lower speeds. Urban Safety Management principles centre on diverting traffic away from streets with high risk to pedestrians and slowing the remaining traffic (IHT 1990).

  A study of child pedestrian accidents in Europe (MVA Limited, 1999) suggests that the lower incidence of low speed limits in the UK, and of measures to reduce speed, contribute to the higher pedestrian casualty rate here. Such rates are particularly marked amongst low income and ethnic groups. Other key reasons for this elevated rate result from differences in the type of road networks, and adjacent housing, in the UK compared to other European countries.


  The quality of life is usually taken to include both objective measures—injuries as discussed above, noise, health and fitness—and the more subjective social issues. The effects of speed are not distributed evenly across society as a whole. The impact on risk for example, per unit of exposure, is greater in some groups than in the population at large. This bears particularly on children, whose cognitive development, including the judgement of speed, is less advanced (Ampofo-Boateng and Thompson, 1991; Sandel 1973) and on elderly people whose declining physical capacity and lower concentration skills make them particularly vulnerable (eg Hine and Russell, 1996; AA Foundation, 1995).

  The speed of vehicular traffic is an important deterrent to cycling (eg Bracher 1989) and, it is widely believed, walking. Since physical activity is important in reducing heart disease and strokes this has health consequences. These have to be balanced against the increased risks of cycling and walking compared with driving. Better knowledge is needed about the possible effect of traffic speed in suppressing walking activity.

  Long streams of fast traffic can cause social exclusion in communities by severance, and by making it difficult for those without cars to achieve adequate mobility (DETR, 2000). There is evidence of perceived improvements in the quality of life where lower speeds have been engineered through traffic calming (eg Danish Roads Directorate, 1987; DTLR, 2001b; Webster, 1998).

  Home Zones, in which road space is shared between motor vehicles and other road users, should impact on local residential urban design in the future. These change the functional use of the streets and engineer more appropriate vehicle speeds. However, the proportion of urban space appropriate for this form of treatment is small.


  Inappropriate and excessive speed are widespread and a range of measures is needed to address them. At present conflicting messages and cues are given to drivers from different elements of the transport system (through road design, speed limit allocations, and vehicle capabilities) and from different practices adopted by Highway Authorities. It will be extremely important to move effectively to a more coherent set of measures, and to reduce these conflicts.

Speed Limits

  Existing limits are in practice applied locally using different criteria (AA Foundation, 2000). The criteria for both built up and rural environments need to be rationalised to consistent national standards. This requires simpler procedures for changing speed limits as conditions change over time, whilst ensuring that, at each change, compatible speed limits, road design and traffic behaviour are achieved. There is substantial evidence of the lack of effect of limits introduced into inappropriate road environments (Finc et al, 1994; Mackie, 1998). But there are also examples where lower speed limits alone have been introduced with apparent benefits in reduced accidents: in Suffolk villages and in Scottish trials; but in both cases extensive publicity or commitment by the local community was required.

  The speed limit regime adopted in some other European countries (notably the Netherlands) is generally lower than in Britain. The norm for roads other than high quality roads is 80kph (50mph). We believe that a similar system relating speed limits closely to road quality should be adopted in Britain. This would require roads to be allocated to speed limit groups based on the accident risk for that type of road in relation to the speed and other mobility and environmental criteria. If an existing road is not up to the required safety standard for the design speed then a lower speed should be adopted until the road can be improved to the appropriate standard. Roads chosen to fulfil a lower traffic function should be designed for lower speeds accordingly.

  To initiate this process it would be necessary to assess the current speeds and design of all GB roads. A programme is already underway—the European Road Assessment Programme—supported by the motoring organisations and several national governments including the UK government, which aims to develop road risk assessments on a common basis nationally and progressively across Europe, and clearly associate risk with road type in drivers' perceptions.

Urban Road Redesign

  20mph zones have achieved considerable success in residential areas in reducing accidents through reduced speeds (Webster and Mackie, 1996). But the more major roads in towns usually need to cope with a wider variety of conflicting functions, requiring treatments to allocate space between competing modes and road users. Completely segregated activities are usually not feasible. Therefore the road design and speed limiting measures need to take account of the risk of the most vulnerable groups. (See Mackie, 1998 and DETR, 2000). The Gloucester safer City project (DTLR, 2001c) provides a good example of how a range of speed reducing measures is essential in wider urban safety management strategies.

Rural Road Measures

  Traffic calming measures within villages, including "Gateway" treatments (Wheeler et al, 1994, 1999) have been used to reduce speeds with the consequence of reduced accidents (Wheeler and Taylor, 2000). Innovative perceptual measures (including roundels and "countdown" markings) have also been trialled with varying success (Barker and Helliar-Symons, 1996; Barker, 1997). Interactive signs can be useful in encouraging lower speeds at bends and junctions on rural roads, and are effective in targeting the higher speed drivers (Winnett et al, 1999).

Measures for Pedestrians and Cyclists

  The accident risk per unit of exposure for pedestrians and cyclists is substantially higher than for car occupants. Whilst complete physical segregation of these modes is usually not possible, it can radically reduce the risk. In most circumstances where full physical segregation is not feasible, it is extremely important to develop solutions for cyclists and pedestrians which reduce their elevated risks. This we see as an essential pre-requiste to achieving safe increases in cycling and walking, and the health benefits that should result.

The Potential of New Technology; Variable Speed Limits

  Technology is available currently that, if sufficiently reliable, could force drivers to adopt lower speeds. The potential acceptability of such a measure raises political issues. Compulsion is most likely to be effective if it is targeted on a minority who fail to comply with generally accepted behaviour, rather than as a tool to change the behaviour of the majority. Shifts in majority behaviour are best effected by engineering and attitude change. However, technology can provide increasingly effective means of presenting information and advice to drivers whilst on the move (for example from in-vehicle devices signalling the speed limit). Such a stream of development, whilst longer-term, could produce large accident savings.

Redesign of Vehicle Fronts

  Injuries to pedestrians struck by cars can be considerably reduced by redesigned car fronts. The optimum testing process to discriminate between those cars giving better protection and those giving worse protection than average has now been rejected by EC, in favour of a voluntary code of practice by car manufacturers. From an examination of the types of injury that might be mitigated under the voluntary system, in relation to the impact speed, we believe this will provide only a quarter to a third of the benefits in fatal accident savings of an EC directive based on the best available test methods. The potential benefits achievable by better car front design are most at impact speeds of around 20 mph. This re-emphasises the importance of reducing the higher speeds in urban areas.

Changing Attitudes

  Many drivers have difficulty in recognising the added risk from increased speed. Mis-perceptions abound. In many respects attitudes and beliefs are similar to those on drinking and driving some 40 years ago. The change in behaviour on drinking and driving which resulted in very large reductions in death and injury came from a series of measures following from evidence produced in the 1960s which clearly showed the relationship between blood alcohol level and accident risk. Now, in 2002, we have a very considerable body of evidence showing the relationship between speed and accident risk.

  This should now be used as the trigger to a process of changing attitudes, driven by a sequence of measures and messages, each one picking up and carrying forward the attitude shift achieved by the previous one. The measures need to be coherent, and to generate a clear perceptual understanding by the driver of the risk inherent in the speed they adopt for a particular circumstance.


  The foregoing underlines the importance of coherence and consistency in the measures adopted by all of the relevant bodies. The essential elements needed for coherent policy and implementation on speed management exist within the system of Local Transport Plans, and the statutory responsibilities of Highway Authorities, including particularly the Highways Agency for the national network, and the Local Highway Authorities for local networks. But speeding, and speed management, are complex issues, and the establishment of clear frameworks for action is still lacking. We should not underestimate the importance of clear and convergent practices in speed management; and we are not convinced there is yet a sufficient single driver of change.

  The substantial body of knowledge now available on the effects of speed and the factors influencing it, place government in a strong position to drive the change required. We believe this is now a pressing need.

  There are in the meantime two particular issues that need to be addressed.

The use of Speed Cameras

  The police should give most emphasis to targeting those speeding offenders whose speed has most effect on accident risk. The objectives of enforcement should be clarified. If there is a very localised speeding problem, then the use of a well-marked camera can be effective in highlighting the local risk. But a key role of enforcement should also be as part of a broader strategy to modify drivers' speed choice to levels which properly reflect accident risk everywhere. In this context, cameras should be used more generally so as to remind drivers of the consequences of adopting excessive speeds. There is no evidence that painting cameras bright yellow will have any effect in additional accident reduction. Moreover, highlighting these cameras through the use of conspicuous colours gives a counter message, that may tempt drivers to adopt higher speeds generally in the confidence that they can brake hard when they spot a camera.

Sentencing and Penalties

  A forthcoming DTLR (2002) report shows excessive speed to be the most common element among dangerous driving cases; but notes a lack of consistency between agencies in how speeding is used to determine dangerous driving. Police clearly feel CPS guidelines are insufficient in relation to speed (and other offences): magistrates feel guidelines are better for speeding than for some other offences. The Home Office review of penalties proposed a two level fixed penalty for those exceeding the speed limit—with a higher penalty for drivers exceeding the limit by more. This is consistent with the effect of speed on accidents shown by TRL research. It also proposed updating penalties over time, which would further support campaigns to reduce speeds.


  This summary presented in this Memorandum, is of necessity, very brief and rests mainly on the evidence cited in the references.

  Taken together, the evidence showing the damaging effects of excessive and inappropriate speed on both the frequency and severity of accidents and on the quality of life is extensive and compelling. We believe that there is a pressing need for actions to reduce such speeds.

Transport Research Laboratory

7 January 2002


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