Variable speeds are the limit I am a law-abiding elderly lady, except when I see no reason for a restriction. I often use the M42 and am frequently frustrated by the seemingly unnecessary speed controls. Yesterday morning was an example: I drove up from London and picked up the northbound M42 (from the M40 junction) at about 11am. The road was virtually empty and weather conditions were fine. Yet the 60mph limit was showing all the time. Who is responsible for activating these signs? The Highways Agency. Some signs are automated according to traffic levels, triggered by pads in the road. Some are manually controlled. Even though they are annoying to the point where most drivers ignore them, they can prevent vehicles bunching. You can't see a reason for them, because traffic continues to flow.