smart motorways

The limits of speed limits


The announcement that the Highways Agency is considering reducing the speed limit on a section of the northern part of the M1 to 60mph received widespread coverage – and some fairly stock arguments.

According to the BBC, the people advocating it claim that it will help to achieve “reduced congestion, increased capacity and improved journey time reliability”, while a spokesman for the RAC fears that “there will inevitably be a negative impact on business efficiency and individual mobility”. So far, so predictable.

The argument that lower speeds may actually mean faster, as well as safer, journeys is complicated by the fact that they are usually introduced on roads which are already highly congested, and in conjunction with other measures. But proponents of such schemes claim that there is data to back up this modern-day application of the adage “more haste, less speed”.

The variable speed limits introduced on several motorways (including the M62 and M42) have helped to produce a 22% improvement in journey time reliability as well as a 55% reduction in personal injury, according to the Highways Agency. But how much of this is due to speed restrictions, and how much to other measures on so-called “smart motorways”, such as the opening of the hard shoulder to traffic at peak periods, is more difficult to establish.

What makes this particular proposal new is that it is the first time a reduction in the speed limit has been proposed specifically for the reduction of emissions and air pollution, rather than being justified with reference to road safety or improved traffic flow. Advocates argue that the restrictions are unavoidable because of EU targets, and than a modest reduction in speed creates significant environmental benefits. As well as improved safety and journey times, speed limits were thought to have created a 10% reduction in emissions in the M42 study.

But it is not just motorways where the trend is gaining momentum. At local levels, too, speed reductions are being widely implemented, with 20mph zones becoming more common in towns and cities. Opponents of Camden Council’s introduction of a blanket 20mph zone last August, including the Licensed Taxi Drivers’ Association and the Alliance of British Drivers, argued then that such a low limit encourages “stop-start” traffic and actually increases pollution and accident rates.

Rural roads, the vast majority of which used to be subject to the national 60mph limit except when passing through villages, or where specific hazards had been identified, have also seen a steady increase in variable limits and restrictions since the requirement for local councils to seek permission for limits from the Department of Transport was removed in 1992.

The depressing likelihood is that the proposed optimal speed limit will probably always vary, depending on whether you consult specialists in  fluid dynamics and traffic flow, road safety campaigners, environmental activists, cyclists, pedestrians and drivers themselves, or those whose business depends upon the road network. It also depends, of course, upon the traffic conditions, state of the road, time of day and a host of other variables.

The best conclusion we may be able to reach without further studies is that in modern cars fuel economy and emissions levels, at least for motorway driving, would usually suggest a driving speed between 50 and 60 mph. The trouble on Britain’s motorways is that, when traffic is light, drivers find that pointlessly restrictive (and potentially dangerous, if the rest of the traffic is moving between 70 and 80mph), and that when – as is so often the case – they encounter congestion, they can only dream of getting anywhere near that speed.





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