As a regular driver on the M25 and a number of other motorways across the UK, I have driven under 100’s of gantries with their large electronic displays indicating the variable speed limit over each lane along with other signs that instruct drivers to stay in their lanes. These displays are intended to help motorists to adjust their driving to improve the flow of traffic and reduce congestion on what is often described as Europes largest circular car park. After years of participating in this process I am using this blog to explain why I think they need to be improved or retired.

The theory is simple. If one car is travelling at say 65mph and another car behind it is travelling at 70mph, when it catches up the lead car, the second car might brake, even moderately which could lead to cars further behind also braking. If the line of cars is long enough, the impact of this braking could lead to almost stationary traffic miles behind, and significant risks of accidents. If the variable speed limit measures the speed of the front car, and sends a message to the cars behind it to set their speed to 65, none of these problems will be created, the road will remain open and everyone will get to their destination safely. At its best this technology can help eliminate small amounts of human error by 100’s of drivers, and avoid a huge cost to the economy as well as possible loss of life.

There are three reasons why I am certain that these systems are failing to deliver with any credibility:

• After years of driving, I have never seen a set of signs that indicate a different speed in one lane from any of its neighbours – if the system of traffic control does what we are told it should, this would be an everyday occurrence.
• The speeds are always rounded to the nearest decadel, I have never seen a speed on any of these header displays without a zero at the end, yet I rarely drive at a speed that comes close to a round number. It is clear that these signs are not capable of measuring or displaying real life information with any precision.
• I have spent too many hours of my life, sat in a stationary car with a sign on the gantry a few yards away that is indicating a speed limit of 30mph, 40mph or even 50mph when it is patently clear that the cars further ahead for 100’s of metres are also stationary. Under these circumstances the signs should show a speed close to zero if they work as they should!

If we are to treat these speed limits seriously, which the law instructs us to do, we need the Highways Agency to fit up to date technology that does what it says on the tin. It is understandable that drivers have a low regard for these signs if they are used in a way which shows a complete disconnection from the traffic on the road below. The risk is that they are seen to be as reliable as the boy who cried wolf with the inevitable consequences!