Should the national motorway speed limit be raised to 80mph? The latest national statistics on vehicle speeds together with police speed enforcement guidance make a compelling case - on uncongested motorways, 57 per cent of car drivers exceed the speed limit, and 20 per cent exceed 80mph. It is rare for the police to prosecute drivers travelling at 80mph.

The average modern car cruises very comfortably at 70mph, and equally so at 80mph. In 1964 when the motorway speed limit was introduced, it was set at the "flat-out" speed of most cars on the road. The AA Trust's European Road Assessment Programme (EuroRAP) also shows that motorways are the safest places to drive. And EuroNCAP crash testing has driven consumers to demand four- to five-star cars that make surviving a crash much more likely than when the 70mph limit was set.

So at one level there is a compelling case for an increase in the speed limit. But of course it is not that straightforward.

It is only possible to exceed the speed limit in uncongested traffic conditions. British motorways are the most congested in Europe and so actually achieving 70mph can be a magic moment for many.

But the most serious concern is that legalising today's unofficial and tolerated 80mph limit would create tomorrow's unofficial 90mph limit. Average traffic speeds would increase, as would accidents and their severity.

The question that has to be asked is: would it be possible to increase the speed limit to 80mph and enforce at that speed? The answer is 'yes', by expanding the use of 'controlled motorway' technologies that have been implemented successfully on the M25 in Surrey.

This system sets the speed limit according to traffic flows and density. In free-flowing conditions the speed limit is 70mph, but when congestion increases the technology switches the limit to 60mph or lower. If traffic flow breaks down completely, or if there is an accident, even lower limits can be set. And with many speed cameras, enforcement is strict, so that very high levels of compliance are achieved.

If the speed limit can be lowered below 70mph in congested conditions, it could just as easily be increased to 80mph when traffic flows are much lighter. There would, of course, need to be a full risk assessment, and some additional crash protection may be needed. Enforcement would be strict, thus avoiding the feared drift to a 90mph limit.

But could we vary speed limits on other roads in response to the risk of accidents? A system that can gauge when schoolchildren are on the road, or when queues are forming around a blind bend, and can impose the right limits, in the right places, at the right time, would be very attractive.

But gantries over every road would be expensive and environmentally unacceptable. So we need a system that gets the speed limit into the car. This is not as far away as it may seem: GPS means that a car can pinpoint its location; digital maps can check the speed limit; a clock knows the time. Put the three together and you are coming close to a dynamic system allocating the best speed to each road in real-time. The system could earn compliance through respect for its accuracy, or could impose it through direct linkage to the controls of the vehicle.

The future must be to have risk-related speed limits that drivers trust; speed limits that can change by time of day and by road conditions; speed limits where the driver can tell the car not to let him break them. Showing that road safety isn't only about slowing down can be a vital first step.

The author is head of road safety at the AA Motoring Trust

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