Is it or isn't it?

Around here there are stretches of motorway where, at times of congestion, the hard shoulder is opened up as a fourth lane.  But the police are now getting concerned that motorists are using the hard shoulder even when it's not officially open.  Ambiguous signing is partly to blame I think, but then there's the question of why, if the hard shoulder can be opened when it suits, it cannot be open all the time.  It's either safe to use or it isn't, surely? The confusion only arises because it's open at some times but not others.  If it were open all the time - no confusion.