Memorandum by University College London,
Centre for Transport Studies (RTS 36)
ROAD TRAFFIC SPEED: UNDERSTANDING AND MANAGING
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
welfarebut the sharing of the costs is unfair in four distinctive
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
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
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
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
speedsbut 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 themdrivers
as well as pedestrianssaw 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 devotedprobably on an EU-wide basisto 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
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 highpartly
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
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 horsebackleading 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
It is right to make this calculation as an input
to decision-makingand 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
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 explicitlyqualitatively
at first pending agreement on quantitative measurement and valuation
(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 appraisalalong of course with the practicalities
of implementation using the available and foreseeable measuresshould
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
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
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,
Mitchell C G B (1993) Influencing speed and
its environmental benefitsvehicle design. PACTS Conference:
Safety, mobility and the environmentstriking 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.