I think the simplest thing which can be done is to paint what might be called the start of the danger zone with a yellow line, like at the edge of a railway station platform. At the rear a hatched area in which the driver should be the headboard side of the pallet (pallets 23 and 24 do cause a problem). Small floor-area tail lifts are probably the biggest hazard for unloading pallets with a pump truck. Apart from this instructions not to walk backwards and to level the floor are the obvious others.