If you're absolutely intent on not exceeding 70, and you wish to overtake traffic travelling at, say, 68, then you are are going to take a very long time to overtake. This is not a safe situation to be in for several reasons. First, the vehicle you are overtaking may not be aware of your presence in his blind spot, in which you will be lingering for an awfully long time. Second, vehicles behind you who, understandably, want to overtake far more decisively than you appear to want to, will be frustrated by the fact that your tardy manoeuvre may actually prevent their overtaking at all. Third, you are taking far too long over a move that is full of potential hazards. If you wish to overtake, do it quickly and decisively. If you can overtake only by taking ages over it because of your speed limit obsession, then stay back and don't overtake. Otherwise, you're a bloody menace to other road-users.