30/09/2011 07:18 BST | Updated 30/11/2011 05:12 GMT

Time for an 80mph Motorway Speed Limit?

The transport secretary has indicated that the motorway speed limit could be raised to 80mph from the current 70mph. The low accident rates for motorways, the average speeds and police speed enforcement guidance do make a compelling case. But would safety be compromised?

On decongested motorways over half of car drivers exceed the speed limit, and around a fifth exceeds 80 mph (Box 1); it is rare for the police to prosecute drivers travelling between 70 and 80mph (Box 2). It can be argued that while the official legal speed limit may be 70 mph, by custom and practice there is widespread adoption and official acceptance of an 80 mph speed limit.

The performance of cars also argues for a higher speed limit. The average modern car cruises very comfortably at 70 mph, and equally so at 80 mph. In 1964 when the 70 mph national motorway speed limit was introduced, it was set at the "flat-out" speed of most cars on the road.

As for safety, motorways are the safest roads because they are designed for safety, as are the modern cars that use them ( Box 3). EuroNCAP crash testing has driven consumers to demand 4-5 star cars that make survivability in a crash much more likely than when the 70 mph speed limit was set.

So, at one level there is a compelling case supporting an increase in the speed limit, but of course it is not quite that straightforward.

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

The greatest concern is that legalising today's tolerated, yet unofficial, 80 mph speed limit would create tomorrow's tolerated and unofficial 90 mph limit. Average traffic speeds would increase, as would accidents and their severity - more people could be killed and seriously injured.

The question that has to be asked is: would it be possible to increase the speed limit to

75 mph or 80 mph and enforce at that speed? The answer is "yes", by expanding the development of "Controlled Motorways" and "Managed Motorways" technologies that have been implemented successfully on the M25 London Orbital Motorway in Surrey, the M42 in the West Midlands and that are being rolled out across the motorway network

On these controlled and managed motorway sections, the speed limit is set according to traffic flows and density. In free flowing congestion free conditions, the speed limit is 70 mph, but when congestion increases the technology switches the speed limit to 60 mph or to 50 mph. If traffic flow breaks down completely, or if there is an accident, even lower speed limits are set automatically or manually. And with the many speed cameras, enforcement is strict so that very high levels of compliance are achieved whatever speed limit at the time.

If the speed limit can be lowered to below 70 mph in congested conditions, it could just as easily be increased to 75 mph or 80 mph when traffic flows are much lighter and driving conditions are good. There would, of course, need to be full risk assessment, and some additional roadside crash protection may be needed. Enforcement would also be very strict, for example driving at 82mph could risk a speeding ticket, so that the feared drift to a 90mph unofficial speed limit would be avoided.

The IAM would like the Highways Agency to pilot an 80 mph speed limit on well established controlled and managed motorways where variable speed limit are already operating to assess its practicality and safety, and drivers' reaction to it. We believe the main benefit would be very high rates of compliance that would engender widespread respect for the higher speed limit, a necessary part of ensuring that speed limits everywhere relate to real risks, and are respected.

Raising the motorway speed limit has been debated for many years. The transport secretary should now publish a consultation with firm proposals