Calibration of the M4 Variable Speed Limit scheme and promising initial results

On behalf of the Welsh Government, Atkins managed the installation of the M4 Variable Speed Limit (VSL) scheme, between Junctions 24-28, to reduce congestion, improve safety and reduce emissions. This innovative scheme uses the Controlled Motorway (CM) concept but has unequally spaced gantries, with some separated by fairly large distances. Where there is a long gap between overhead gantries, post-mounted MS4 signs are used to provide repeater speed limit information to drivers. This allows significant cost savings from reduced infrastructure, while retaining the benefits of using speed control to reduce congestion. However it means the Controlled Motorway algorithm was being used in a new and previously untested way, so it was important to ensure that its implementation would be successful. To do this, modelling was performed by Atkins to check the scheme would have a positive impact and verify the chosen CM algorithm parameters before they were used in the live system. The modelling was a success, and the live system was activated in mid-November 2011 and calibrated for a month in November-December 2011. Initial feedback and findings are very promising; drivers and control room operators believe the system is an improvement, and there are initial indications of congestion reduction.