Highways England sometimes vary the speed limit on certain sections of motorway to steady the flow of traffic and reduce traffic congestion.
Variable speed limits can be set at busy times, in conjunction with a Red X sign or to manage a hazard, broken down vehicle or incident. They can also be automatically triggered by sensors which monitor traffic flow.
A speed limit displayed within a red circle is legally enforceable so if you go over the signed speed limit, you are breaking the law and could receive a fine. When speed limits are displayed, they remain in place until notified that they have ended by using the National Speed Limit sign, or until a different speed limit is displayed.
If no speed limits are display then the carriageway is National Speed Limit. Variable Speed Limit Cameras or HADECS operate within the Smart Motorways and are either positioned to the side of the overhead gantries or in the back of them.