Select Committee on Environmental Audit Written Evidence

Memorandum submitted by David Starkie

  1.  The Committee has invited evidence in relation to the above Inquiry and is particularly interested in a number of specific issues. This memorandum is focused on its strategic issues and, in particular, on bullet points 4 and 5 listed in the press release.

  2.  The dominant source of transport emissions is road traffic and, because of this, attention has focused on changing the fuel mix of vehicles and reducing their average fuel consumption as a result of developments in vehicle technology. The Department for Transport's low carbon vehicle strategy is such an example. What is overlooked is the important relationship between vehicle speed and fuel consumption and hence between speeds and emissions of carbon. The proposition in this memorandum is that limiting or eliminating excessive traffic speeds, and especially by achieving compliance with national speed limits, would make a major contribution to meeting the Kyoto targets on carbon emissions.

  3.  The reason why compliance with speed limits, and particularly with the maximum limit of 70mph, would make a major contribution to reducing carbon emissions is two-fold.

  4.  First, fuel consumption, and hence greenhouse gas emission, of vehicles is basically u-shaped with respect to speed. The economical speed for most vehicles is between 50 and 60 mph. Increasing speed beyond this level increases fuel consumption at a progressively faster rate, so that a vehicle travelling at, say, 90 mph is emitting far more pollutants than a vehicle travelling at 70 mph.

  5.  Second, speeding on Britain's roads is endemic, especially on motorways and trunk roads, which account for about one-third of total vehicle miles. The table below illustrates this situation with respect to driving speeds on motorways and dual carriageways over the 10 year period 1994-2004. I have compiled the table by adapting figures in the Department for Transport's annual publication Vehicle speeds in Great Britain. These figures are based on survey sites where the traffic is likely to be free flowing but slower recorded speeds during the morning and evening peak periods suggest that this is not always the case. Consequently, the survey data probably under-estimates free flow speeds.

  6.  From the table it can be seen that car speeds have been steadily increasing over the period such that by 2004 the average speed of cars on motorways exceeded the national speed limit and on dual carriageways was almost identical to the national speed limit. On motorways well over half of cars were exceeding the speed limit, whilst on dual carriageways almost one half exceeded the limit. Importantly, a large number of cars (and light goods vehicles) exceeded the limit by more than ten miles per hour. This proportion has also grown over time with the result that on motorways nearly one in five cars exceeds eighty miles per hour and thus emits disproportionately large quantities of carbon.




Average speed (mph)
Speed limit (mph)
Per cent over limit
More than 10mph over limit

56 (51)
19 (17)
Dual Carriageways
48 (44)
14 (13)

  Source: Adapted from Vehicle Speeds in Great Britain 1994 and 2004 editions.

Statistics for light goods vehicles shown in parenthesis.

  7.  Increased enforcement of existing limits using traditional policing methods is not an option, if only because the police give speed enforcement low priority and are not willing to commit more resources to the task, given what they perceive as the high opportunity costs of doing so.

  8.  In urban areas greater use is being made of technology, particularly speed cameras, to deter speeding. It is doubtful, however, that the approach of using vehicle recognition technology would have much success in reducing speeds on motorways and trunk roads unless the commitment of resources was very considerable. This is because exceeding the 70mph limit is so wide-spread that changing behaviour will be very difficult without a very large number of prosecutions (which would probably be unacceptable politically). The circumstances might suggest that the maximum speed limit should be increased and coupled with stronger enforcement, but it is not at all clear that this would eliminate a tail of vehicles exceeding a revised limit. It is most likely to lead to an overall increase in emissions (and accident rates).

  9.  Another approach is to extend the application of speed-limiters to vehicles. Mechanical speed-limiters have been fitted to all new heavy goods vehicles (with a maximum weight in excess of 7.5 tonnes) since 1992 and since that date most buses and coaches have been fitted. This has led to a significant drop in the number of such vehicles exceeding the motorway speed limit with a consequential saving in emissions. The technology has developed since the early 1990's so that electronic devices are now probably cheaper and more effective (the technology is similar to that incorporated in vehicle cruise control).

  10.  There are a number of options for extending the application of speed limiters, which present themselves. The generic options are:

    —  to incorporate speed limiters in newly registered vehicles after a specific date; and

    —  to retrofit tranches of the existing national stock of vehicles (not already speed-limited), in accordance with a roll-out programme.

  11.  Because parts of the existing stock of vehicles (HGVs and buses and coaches) are already fitted with speed limiters, there are also options in terms of which other classes of vehicle might be (electronically) speed limited. My order of preference would be as follows:

    —  Medium weight goods vehicles: under current regulations large commercial vehicles of up to 7.5 tonnes are not fitted with speed limiters and are also free to use the third lane on motorways. Because of the vehicle mass involved, high-speeds by these vehicles are associated with particularly high fuel consumption.

    —  Taxis and mini cabs: these are essentially a form of public transport and are increasingly used for long-distance journeys, particularly to airports. (I also have personal experience of an unpleasant journey in a taxi at over 90 mph between Gatwick and Crawley).

    —  Light goods vehicles (including the white van brigade).

    —  Company cars: surveys have shown that company motorists admit to speeding more than any other group of drivers and have more points on their licences (see RAC Report on Motoring, 2001-02).

    —  Private cars.

  12.  There are most likely to be objections to such a proposal from those of a libertarian disposition. However, it is difficult to present a civil liberties argument in relation to most company cars and vans that are driven on behalf of third parties. The incorporation of all but the last category of vehicle in the above list would also address, to some extent, the issue of employer liability and manslaughter. It would allow the employer to claim that s/he had taken reasonable steps to ensure vehicles were driven at the maximum legally permitted speed limit.

  13.  In addition to the important emission benefits that the proposal would introduce, it would also have considerable benefits in relation to road safety.

  14.  There are also likely to be benefits in relation to road capacity. Removing the tail of vehicles exceeding the limit is also likely to lead to smoother flows of traffic because vehicle speeds will be more convergent; (this is what variable speed limits on the M25 for example try to achieve). It is possible that a reduction in accidents that could be expected to accompany the elimination of excessive speeds, and that would otherwise have interrupted the traffic flow, might be sufficient to offset the reduction in maximum speeds; average journey times might be affected little if at all by the speed constraint.

  15.  In conclusion, I am of the view that extending electronic/mechanical speed limiters to more groups of vehicles would make a major contribution to reducing emissions from road vehicles. This is because of the disproportionate use of fuel as vehicle speeds increase progressively beyond 50/60 mph, the very high proportion of vehicles exceeding national speed limits on motorways and trunk roads, and the fact that these road categories account for about one third of all vehicle miles. Precise estimates would require technical study but I would hazard a guess that if all vehicles were to comply with the 70mph national speed limit, this alone would reduce total road vehicle emissions by 5 to 10%. The object of doing so should form part of the national strategy for achieving the carbon reduction targets.

February 2006

previous page contents next page

House of Commons home page Parliament home page House of Lords home page search page enquiries index

© Parliamentary copyright 2006
Prepared 7 August 2006