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

Supplementary memorandum by the Slower Speeds Initiative (RTS 34A)


  "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."[107]

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 distributed.

  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.''[108]

  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 Policy[109] using crashes, travel time, fuel costs and non-fuel vehicle operating costs.

  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''.[110]

  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 cameras.

  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

108   Allsop, R (1990) Aspects of Speed in Relation to Traffic Safety, quoted in Kallberg, V-P (1997). Back

109   Plowden, S and Hillman, M (1996) Speed Control and Transport Policy, London: Policy Studies Institute. Back

110   Allsop, R E (1998) Summary of Research Area 1: Basis for appraisal of effects of different levels of speed, Working Paper R1.3.1, MASTER. Back

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