@I don't get it
Consider a situation where, for some reason, traffic bunches up causing cars to come to a halt.
Now imagine that that creates a "block" of stationary traffic 100 metres long.
If the cause of the original blockage then goes away, traffic can leave the "block", however if traffic is *arriving* at the block at the same rate as traffic leaves, the block will remain.
If, however, you slow down the traffic arriving at the block, it will dissipate.
Of course this relies on people believing and obeying the "slow down" signs, however *that* relies on the Highways Agency *switching off* the signs when they are no longer needed. (For example a few days ago I was on the A3 coming out of Portsmouth and there was a big warning sign saying "Queue Ahead 40mph", but there was no queue to be seen.
These "cry wolf" signs left running without purpose don't benefit anyone.