Average speed limits do not exist but Highways England uses variable speed limits on smart motorways to smooth traffic flow, reduce congestion and enhance safety. The variable limits are set locally in response to traffic flow levels or to help manage incidents.
The Department issued revised guidance in January 2013 aimed mainly at local traffic authorities who are responsible for setting speed limits on local roads. It includes guidance on the use of variable 20 mph speed limits with electronic signs. It has been designed to help explain to everyone why and how local speed limits are determined. This guidance was revised following full public consultation in Summer 2012 and is available online on GOV.UK