Smart Motorways Not So Smart

Anyone who drives around the M25 will have noticed that the speed limits set on the overhead gantries appear to bear no relation to the traffic volumes. Speeds are often set at 60, 50 or even 40 for no obvious reason. Other “Smart Motorways” across the country show the same problem, and the result has been a large increase in the number of people fined (or diverted to speed awareness courses) for exceeding the set speed limit. The number of infringements doubled last year according to the Times.

The justification for smart motorways was that they can reduce congestion by smoothing the traffic flows and help to maintain safety. The speed limits are set partly by automated systems that measure the speed of traffic, but apparently they are also set “pre-emptively” by staff where known congestion is likely to occur at busy times. But as traffic volumes can be unpredictable this sometimes results in lower speeds being set than is appropriate – you can frequently see this around the western side of the M25 around London airport and on the eastern side near the Dartford Crossing.

In addition it is known that the odd particularly slow moving vehicle can result in the speed limit being reduced by the automated system. In other words, the “smart” system is not at all intelligent.

However Highways England is now undertaking a comprehensive review of variable speed limits on motorways. Surely it would be better to simply have an advisory system to tell drivers that there was congestion ahead so that they can slow down and avoid the “stop/start” problem that reduces traffic flows?

At present you have a dumb system instructing intelligent humans (which they mostly are) with the result of needlessly slower traffic speeds and drivers being caught out by unexpected changes in the limits or signs they may not have seen or noticed.

Roger Lawson



1 thought on “Smart Motorways Not So Smart

  1. When the variable limits on M-25 were only advisory years ago, they were great, and they sharply reduced the rubber-bans start/stop queues. A driver could roughly follow the limits when the conditions at their precise location made that smart, but could speed up in gaps where they were not correct for their lane and close by traffic. As enforced limits, they are a vicious for-profit racket.

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