Understanding ‘Hang-back’. 
Hang-back is the name given to the road position that you should adopt prior to making your final commitment to overtake. This position will provide a good ‘zone of vision’ which will in turn assist your planning for the overtake.
The hang-back position (2) is substantially closer to the vehicle in front than a normal safe following position (1) – it will normally approximate to your thinking distance.
This position eliminates the need to close up a lot of space immediately prior to overtaking and enables the manoeuvre to be completed quicker and over a shorter distance. You must, however, compensate for the lack of separation distance by increasing your forward vision.
While ‘hanging back’, you may need to adjust your road position to the right or left in order to maximise forward vision, for example, if the road curves to the left ahead you may get a better view by moving to the left and looking beyond the ‘target’ to the nearside.
The hang-back position should not be adopted indefinitely. Driving in this position places increased stress levels on the drivers of both the overtaking vehicle and the target vehicle.
If an overtaking opportunity does not seem to be likely in the short term, a normal two-second following distance must be re-established.
