When the early signs of congestion are detected on the M42 Active Trafc Management scheme, the control centre operator and automatic system will change the signs and signals to alert road users to the conditions ahead.
The system automatically calculates the best speed to keep the trafc owing and then this is displayed on the speed limit signals overhead.
The control centre can also open the hard shoulder as an extra running lane to reduce the impact of congestion by providing extra space for the trafc. This will help to keep the trafc owing and minimise delays.
'Theoretically a three-lane motorway reaches capacity at around 5,400 vehicles per hour, ' says Grant. Very roughly speaking, when we get to 5,000 vehicles per hour a light comes on in the control centre saying we are getting near to capacity. The variable speed limit signs are automatic, the hard shoulder is manual.' Before the hard shoulder is opened, a series of safety checks are performed. 'We do a debris sweep of the hard shoulder with a trafc ofcer and carry out visual checks. We also have sensors in the hard shoulder every 100m that detect the presence of vehicles. All of that takes place before we open it, ' says Grant.