To ask the Secretary of State for Transport, for what reason variable speed limits of 60 mph were in force on the M1 motorway at 3.30 pm on 27 November 2014 when traffic flows northbound were relatively light; what measures are in place to prevent those operating the variable speed limits anticipating heavier traffic when, in fact, it does not materialise; and if he will make a statement.
The Highways Agency has systems installed across the motorway network which measure the speed and volume of traffic flows using inductive loops buried in the carriageway or roadside radar. When the number of vehicles rises above certain predefined thresholds and/or if the speed of traffic drops below a set level, progressively slower speeds are set on the motorway signs and signals. In some locations these are advisory speed limits and act as a safety measure to gradually reduce the speed of traffic to avoid sudden braking and bunching, which is a notable cause of accidents. It also helps to protect the back of any queue that may have formed. On ‘Smart Motorways’ the systems trigger mandatory speed limits that provide the safety benefits already mentioned but also maintain traffic flows and thus reduce overall journey times.
Even though the safety and congestion benefits of this system have been proven, there are circumstances where the systems can be triggered to show reduced speeds even when traffic flows appear light. For example, a slow moving abnormal load or heavy equipment on an inclined section of motorway. On occasions the systems are so effective in managing traffic flows that the expected congestion from traffic flows is prevented and thus road users see the warning signs and reduced speeds but do not see the queue/congestion. The systems continually monitor flows and will reset when conditions allow.
The Highways Agency’s Regional Control Centres maintain a 24/7 overview of the network and can see where the automated systems are generating queue messages and/or reduced speeds. Extensive CCTV coverage, particularly on sections of Smart motorways (such as the M1, M6, M25, M42 and M62), means that operators can review conditions and override settings according to the situation.
I have asked the Highways Agency for a report on the specific circumstances raised in the question.