I wonder if any research has been conducted into whether or not these signs become something of a self fulfilling prophecy? I notice them on the M6 on Friday evenings when driving up to Telford, and I have noticed traffic slowing down as you would expect...Thing is, when no jam or queue appears, those people have slowed for no reason...And slowing for no reason is one of the causes of the phantom traffic jam since the braking effort has to be expelled from the system somewhere.
I appreciate this isn't in relation to the OP, but the self-fulfilling prophecy is supposed to be the other way round (probably should be called something like a self-cancelling prophecy).
If you have a phantom jam, it reduces the road capacity (typically by about 1/3) and the jam exists (and moves backwards) for as long as there are more vehicles entering the jam than can be released from the jam.
If you can create a gap in the traffic so that there is even just 10 or 15 seconds where the rate of rate of entry of vehicles is less than the capacity of the jam, then the jam will simply disappear. The trick is to try and create that gap without creating a jam in the process. This is most effectively done with gantry signals that can be seen from a distance rather than reacting to the brake lights of the vehicle in front of you.