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

Memorandum by University College London, Centre for Transport Studies (RTS 36)


  Speed brings great benefits in the form of shorter travel times for people and goods (and sometimes enhancement of the experience of travel) at substantial cost in terms of accidental death, injury and damage, environmental damage, loss of quality of life and some other aspects of transport policy.

  We all share in the benefits from speed (even those who rarely or never use a car) and we all bear some share of the costs. The benefits from speed are probably neither more nor less fairly shared across society than many other kinds of welfare—but the sharing of the costs is unfair in four distinctive ways:

    —  many of the costs are not borne by those who benefit most directly from speed;

    —  there is a tendency inherent in the road traffic system for all of us to drive faster than is good for ourselves or society;

    —  massively disproportionate costs are borne by those who are killed or seriously injured in accidents and by their close associates; and

    —  costs of global warming and some other environmental damage will be borne by our children and our children's children rather than by ourselves.

  All this means that responsible government of any party must seek to influence speed and in many respects to moderate it.

  This memorandum tries to summarise the impacts of speed of traffic on any particular road; the rationale for speed management on particular roads; some wider impacts of speed and implications of speed management for current transport policy; and some issues concerning implementation of speed management in the present policy context.


  The impacts of the speed of traffic on a particular stretch of road can be summarised as follows:

    —  The risks of accident, injury and death rise with speed at all speeds, they rise faster as speed increases, and the more severe the consequences being considered, the faster the risk rises with speed (see eg Anderson and Nilsson 1997).

    —  Journey-time falls with increasing speed, but falls more slowly as speed increases.

  The diminishing returns in terms of journey-time from increasing speed and the ever more rapidly rising risk together imply that there must at some intermediate value of speed be a balance between time saved and extra risk incurred. But just where the balance lies is also influenced by environmental impacts per vehicle-kilometre travelled, which also depend upon speed (see eg Mitchell 1993).

    —  Emissions of oxides of nitrogen, which contribute to acid rain, increase with speed at all speeds.

    —  Emissions of particulates and carbon monoxide, which contribute to respiratory problems, both increase with speed at higher speeds, but at lower speeds they decrease as speed increases.

  Recent cars equipped with catalytic converters have lower levels of emissions after the first few kilometres of travel from cold than older vehicles, but emissions of oxides of nitrogen and carbon monoxide still increase with speed at higher speeds.

    —  Emissions of hydrocarbons, which contribute to formation of smog, decrease with speed up to quite high speeds and then increase only slightly with speed.

    —  Emissions of carbon dioxide, which are proportional to fuel consumption and contribute to global warming, decrease as speed increases at lower speeds, but increase with speed at higher speed.

    —  Traffic noise experienced in the neighbourhood of the road decreases as speed increases at lower speeds, at which it consists mainly of engine and transmission noise, but increases roughly as the square of speed at higher speeds, at which it consists mainly of tyre noise.

  Broadly speaking, all the impacts except journey-time argue against the highest speeds and all except oxides of nitrogen and risk of accident, injury or death argue against the lowest speeds—but the decrease in risk with decreasing speed is modest at speeds below about 30kph or 20mph.


  Choice of speed in the prevailing circumstances is in many respects a matter for the individual driver or rider, and in a free society it is an important precondition for seeking to regulate or otherwise limit drivers' and riders' freedom of choice of speed to have clear grounds for doing so.

  Prevailing speeds are determined by the choice made by drivers and riders on each stretch of road as they find it. The drivers and riders get much of the benefit immediately for themselves and their associates in terms of earlier arrival (and possibly the pleasure of going faster). They do bear some of the costs themselves (in increased fuel consumption, wear and tear to their vehicles, and risk of accidents and their consequences for themselves), but they are known to under perceive these costs. They do not themselves bear any of the human consequences of accidents for others than themselves and their associates, or much of the damage to the environment, or any of the damage to the quality of life in the areas through which they drive. This is why there is an inherent tendency for all of us to drive faster than is good for ourselves or society, and why speed management is largely concerned with moderating the currently chosen levels of speed.

  Research forming one part of the wide-ranging EU-funded project MASTER (managing the speeds of traffic on European roads) (Kallberg et al 1998 a,b) interviewed drivers and pedestrians in various kinds of location on the roads of six EU Member States from Spain to Sweden about their views on current levels of traffic speed (Risser and Lehner 1998, summarised by Allsop 1998). The results were broadly consistent across all six countries. Half the drivers and three-fifths of the pedestrians thought speeds were generally too high. Most of them—drivers as well as pedestrians—saw speed as a principal source of danger in their use of the roads, and recognised a range of advantages in speeds being lower.

  Thus, notwithstanding current choices of speed, the climate of public opinion may well be favourable to speed management policies and measures for moderating speed, and it is reasonable that such policies form an important element in the Government's road safety strategy to 2010, Tomorrow's Roads, Safer for Everyone.

  Research in the project MASTER showed that existing information is more than sufficient to underpin the initiation of policies for speed management on roads of various kinds, but it also showed that as such policies are developed over the coming years they will need to be further underpinned by more up-to-date and situation-specific quantification of effects of speed upon accident risk, exhaust emissions and traffic noise.

  The research showed (Allsop 1998) that extensive and underused quantitative and qualitative knowledge about these relationships exists, but that in order to make this accessible to and usable by speed management policy-makers and those applying the policies to particular roads, considerable effort needs to be devoted—probably on an EU-wide basis—to marshalling the information that exists about environmental impacts of speed and presenting it in directly relevant forms. Predictive relationships between speed, traffic flow, road features and accident occurrence have already been considerably enhanced by work in Britain (Taylor, Lynam and Baruya 2000).


  The following substantial synergies can be seen between achievement of generally lower levels of speed (on all kinds of road except heavily congested motorways and main urban and rural roads, where the balance of advantage almost certainly lies in trying to raise speeds at least somewhat) and other aspects of current transport policy as set out in A New Deal for Transport, Better for Everyone.

    —  The motivation to buy unnecessarily powerful cars would be reduced, thus reinforcing fuel price policy in encouraging movement towards a more fuel-efficient vehicle fleet.

    —  The motivation to use the car for very short journeys from home would be reduced, because much of the length of these journeys would be at speeds typically less than 20 miles/h in residential and similar built-up areas. This would help with traffic reduction in residential areas and to some extent on urban main roads, and encourage walking and cycling for the shortest journeys and cycling and the use of public transport for somewhat longer journeys.

    —  Increased journey-times by car for longer, mainly inter-urban, journeys would contribute to traffic reduction by tending to reduce the lengths of journeys that people would choose to make, through the reverse of the traffic-inducing effect of road construction.

    —  The journey-time advantages of rail over car for particular kinds of longer-distance journey would be increased, thus making a further modest contribution to traffic reduction by shifting some such journeys to rail.

    —  Even on the kinds of road where the balance of advantage would lie in increasing speed, this would reinforce the argument for demand management to reduce wasteful congestion on these roads, and thus for traffic reduction where the resulting gains would outweigh the loss of benefit from vehicle use that would be diverted or foregone.

  In all of these ways, the achievement of generally more moderate speeds would contribute to sustainability and through traffic reduction also to danger reduction over and above the reduction in risk that is to be expected on any particular road by moderating speeds on that road in isolation.


  It is relevant to the Government's road safety strategy to 2010 that moderating speed reduces the number and severity of accidents on all kinds of road. Reducing the speeds of the fastest few per cent of drivers on any given stretch of road is likely to bring the greatest accident reduction benefits. Both in towns and in the country reducing speeds on roads where they are already relatively low may well reduce accidents by a higher percentage than doing so on roads where they are high—partly because the lower speed roads are the ones where more pedestrians, cyclists and horse-riders are about. But on roads with higher speeds the proportion of accidents that are fatal or serious is higher.

  In the context of international commitments to reduce carbon dioxide and more local concerns about air quality and noise, reducing higher speeds reduces not only carbon dioxide but also emissions of oxides of nitrogen and particulates. It also reduces tyre noise. Reducing lower speeds can also reduce emissions of oxides of nitrogen but can increase those of carbon monoxide, hydrocarbons and particulates, as well as carbon dioxide and engine noise. But these adverse effects can be at least substantially offset by achieving smoother movement of traffic where speeds are lower, thus reducing the need for, and sharpness of, braking and acceleration. Air quality effects will decrease in future as low-emission vehicles increase as a proportion of the fleet, but carbon dioxide produced by motor traffic can be reduced only by reducing the amount of fuel that is used.

  Reducing speed can reduce feelings of danger from traffic and consequent community severance and its contribution to social exclusion. It can help to return streetspace in towns and villages and the use of country lanes to people on foot, on bicycles and on horseback—leading to improvements in health through increased walking and cycling.

  It must of course be recognised that moderating speeds to reduce the costs of speed will also reduce the benefits from speed because it will increase travel times (and indeed the lower the speed is to begin with, the greater will be the effect on travel time of a given reduction). There are accepted ways of attaching monetary values to these reductions in benefit, as well as to the gains that come from reductions in accidental death, injury and damage, for consideration along with the other monetary costs of implementing speed policy in estimating its effect on the economy.

  It is right to make this calculation as an input to decision-making—and to consider the resulting numbers alongside the environmental impacts, the less quantifiable economic effects, and the effects on accessibility and integration, in appraisal of speed management in common with other aspects of transport policy.

  However, it is also important to remember that these monetary calculations and other elements of the New Approach to Appraisal are an aid to judgement in decision-making and in no way a substitute for it. In the context of speed management, a number of reservations should be taken into account when interpreting the Appraisal Summary Table.

    (a)  Casualty reduction is not the only safety effect: danger reduction should also be included explicitly—qualitatively at first pending agreement on quantitative measurement and valuation of it.

    (b)  Calculation of net present value and benefit/cost ratio in monetary terms addresses the question of economic efficiency but pays no attention to equity, whereas, as already mentioned here, the inequity of incidence of costs of speed is a major issue in speed management. Many would feel that where large numbers of people giving up very small amounts of benefit can save small numbers of people from having to bear massively disproportionate losses, it is reasonable to expect those small amounts to be given up even if in aggregate their value were somewhat to exceed that of the losses from which the small number are spared.

    (c)  In particular, when the logic of net present value and benefit/cost ratio has been so conspicuously disregarded in the underallocation of resources to casualty-reducing road safety engineering, resulting in massive underinvestment, it would be ironic indeed if current valuations of small losses of time by the many, relative to prevention of death and lifelong disability for the few, were to be used in superficially rigorous but spuriously narrow justification for reluctance to act vigorously to moderate speeds.

  Used with proper judgement, however, current techniques of appraisal—along of course with the practicalities of implementation using the available and foreseeable measures—should help the Government to identify a fair and achievable balance between the effects of speed for which to aim through speed management.

  But another judgement that the Government will have to make in their determination to influence speeds is how far and how fast it is effective to try to move ahead of current opinions and behaviour on the part of the drivers who will continue to determine what will be the prevailing speeds in the future. However strongly those who don't drive may feel, and however strongly drivers may feel when they are themselves walking or cycling or riding a horse or sitting in their home or garden, or even discussing the issue of speed in principle, it will be what drivers actually do when they are at the wheel that will determine the outcomes of speed management. Government often has the job of moving ahead of relevant opinion, and bold government will move as far ahead as it can carry the relevant people with it. The challenge to the Government in relation to Road Traffic Speed is to judge rightly just how far that is.

Richard E Allsop

January 2002


  Allsop R E (1998) Summary of Research Area 1: Basis for appraisal of different levels of speed. MASTER Working Paper 1.3.1. Brussels: CEC DG-TREN.

  Andersson G and G Nilsson (1997) Speed management in Sweden. Linköping: Swedish National Road and Transport Institute VTI.

  Kallberg V-P, R Allsop, H Ward, R Van der Horst and A Varhelyi (1998a) Recommendations for speed management strategies and policies. MASTER Deliverable D12. Brussels: CEC DG-TREN.

  Kallberg V-P, H Ward, R Allsop, R Van der Horst and A Varhelyi (1998b) Strategies and tools for speed management on European roads. European Transport Conference, Loughborough, September 1998.

  Mitchell C G B (1993) Influencing speed and its environmental benefits—vehicle design. PACTS Conference: Safety, mobility and the environment—striking the balance, London, March 1993.

  Risser R and U Lehner (1998) Acceptability of speeds and speed limits to drivers and pedestrians/cyclists. MASTER Deliverable D6. Brussels: CEC DG-TREN.

  Taylor M C, D A Lynam and A Baruya (2000) The effects of drivers' speed on the frequency of road accidents. TRL Report 421. Crowthorne: Transport Research Laboratory.

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