Erroneous speed limit sign at base of on ramp at M6 Junction 14

Every morning when joining the motorway at M6 J14 southbound the variable speed sign at the bottom of the ramp is showing 50, so my assumption is that the carriageway above has been limited to 50mph, but every time I join the carriageway the other vehicles appear to be moving at a quicker speed. As the last variable speed sign I have seen was showing 50 it is irresponsible and potentially illegal to travel at a higher speed until I pass another sign showing a different speed. I have therefore encountered numerous occasions when traffic on the main carriageway has pulled into the left lane behind me (believing that I am travelling at the same speed as them), they then have to brake / slow down and then start adjusting to pull into lane 2 to go past me - often, at which point I then reach the first overhead gantry which tells me that (once passed it) I can travel at 60mph - so I speed up to 60 - much to the confusion and sometimes anger of the vehicle which has had to slow down and is trying to overtake as I adjust my speed to what I suspect the rest of the carriageway was doing in the first place. I would suggest that it would be far safer to display the speed at the bottom of the on ramp as the same as is applicable to those already on the main carriageway.