Supplementary memorandum by the Slower
Speeds Initiative (RTS 34A)
SPEED ASSESSMENT FRAMEWORK
"Driving speeds significantly affect practically
all key attributes of road transport. Therefore, in pursuit of
a more efficient, safe and environmentally acceptable road transport
system, we must be able to assess and predict the various effects
of changes in driving speeds."
Why a Speed Assessment Framework is Needed
The impacts of speed are wide ranging. Speed
is the single most important factor in determining crash frequencies,
severities and their subsequent costs to society. Travel time,
vehicle operating costs, pollution and noise are all direct functions
of speed, while less direct impacts include intimidation, accessibility
and location of development. The impacts of speed are unequally
Speed limits have historically been determined
by driver acceptability or by reference to a combination of factors
such as road geometry and traffic flows where the empirical basis
is unclear. Speed limits have not been determined on the basis
of the wider impacts of speed. They cannot be demonstrated to
be fair or efficient for society in general and for an integrated
and sustainable transport system in particular.
``There is no reason why the factors that lead
drivers to choose particular speeds should result in levels of
speed that are preferable from the point of view of society as
a whole, because some important gains and losses to society resulting
from vehicles being driven at different speeds are not experienced
individually by their drivers.''
The Initiative advocates an objective, transparent
and accountable procedure for setting speed limits.
Determining Optimal Speeds
Optimal speed limits for main roads outside
towns were calculated in 1996 in Speed Control and Transport
using crashes, travel time, fuel costs and non-fuel vehicle operating
The authors first calculated what the costs
of travel on each class of road would have been (using 1993 data)
if the present speed limit or some lower one had been enforced
and everyone had continued to make the same journeys. They then
made calculations of what optimal speed limits would be assuming
that increased travel times would lead some people to modify their
behaviour. Their calculation showed that in both cases optimal
speed limits would be lower than at present. The range of revised
speed limits is shown in the table:
The authors recommended large-scale trials of
a range of lower speed limits to determine what the precise limits
should be and to demonstrate the impacts and benefits of lower
speeds to the public.
A key area of work undertaken in the MASTER
project (completed 1998) was determining acceptable ranges of
speeds. An assessment framework was developed ``to assist decision-makers
in choosing an acceptable range of speeds for any given category
of road in any particular region as the aim of speed management
and as an input to the development of speed management strategies''.
The MASTER framework enables assessment of the
effects of speed, their magnitude and distribution, in terms of
crashes, travel time, emissions, vehicle operating costs and other
impacts relevant to the user. The framework allows users to choose
specific impact functions (how crashes or emissions vary with
speed). The minimum data requirements are length of road section
or network, traffic volumes, mean speed of traffic before and
after the speed change and the expected change in number of crashes.
The outputs of the assessment process include
quantitative impacts and their monetary values where applicable,
qualitative and distributional impacts and sensitivity tests.
The Government has acknowledged that a new hierarchy
for rural roads must be progressed in connection with a speed
assessment framework. We recommend that a speed assessment framework
should be used for determining all speed limits.
Andrew Bennett raised the question of how long
it would take to get ``spy-in-the-sky'' systems in place to control
vehicle speeds. The Slower Speeds Initiative believes that a driver-operated
variable speed limiter could be introduced more rapidly than an
external control system for two important reasons. The first is
that a driver-operated limiter would be more acceptable to drivers.
Research undertaken in the MASTER project supports this view.
Of four systems of Advanced Transport Telematics test, automatic
speed control was the least liked. (See Kallberg, V-P et al, 1998,
Recommendations for Speed Management Strategies and Policies,
Deliverable 12, MASTER.) The second is that the technology, based
on current cruise control technology, is already available.
With a driver-operated speed limiter the driver
would simply set the control at the desired point. Colour coded
lights, visible from outside the car, would indicate the speed
to which the vehicle had been limited. Although this system would
still require speed limit enforcement, it would make it very much
easier. In addition, it would provide one warning of the degree
represented by a vehicle to all other road users.
There is no technical reason why all new cars
could not be manufactured with speed limiters straight away. Fitting
a driver-operated speed limiter to a car during manufacture would
probably add about £40 to its cost (excluding the lights).
Existing cars could be retrofitted at a cost of around £250
per vehicle (not including labour). It would thus be much easier
to introduce legislation making speed limiters compulsory both
for new and existing cars.
With appropriate legislation, driver operated
speed limiters could be introduced much more rapidly than a system
dependent on external controls and costly technology, thus saving
thousands of lives.
(See Plowden, S and Hillman, M, 1996, Speed
Control and Transport Policy, London: Policy Studies Institute,
for a fuller discussion of driver-operated speed limiters).
Clive Betts asked whether any research had been
undertaken into the effectiveness of covert versus overt speed
The DETR commissioned research, now posted on
the DTLR website, into driver response to speed cameras. (See
DTLR, under Road Safety Research, Corbett, C and Simon, F, 1999,
``The effects of speed cameras: How drivers respond'', Brunel
University Centre for Criminal Justice Research.) The study recommended
a policy of ``reducing the visibility of roadside cameras'' linked
to publicity campaigns and ``combined speed limit and camera warning
signs in the target area.'' Their recommendation was based in
part on interviews with those drivers who are the most likely
to break the speed limit, by the greatest amount, and who are
also the most likely to crash.
107 Kallberg, V-P (1997) Framework for Assessing the
Effects of Speed, Working Paper R1.2.3, MASTER (Managing Speeds
of Traffic on European Roads). Back
Allsop, R (1990) Aspects of Speed in Relation to Traffic Safety,
quoted in Kallberg, V-P (1997). Back
Plowden, S and Hillman, M (1996) Speed Control and Transport
Policy, London: Policy Studies Institute. Back
Allsop, R E (1998) Summary of Research Area 1: Basis for appraisal
of effects of different levels of speed, Working Paper R1.3.1,