I did some work for Royal Mail a few years ago, and their standard load unit was a specially-designed roll-cage (originally made to fit on mail trains). So load-unload times for double-deckers will be pretty good (better than two single-decked vehicles if you include time to drive in/out of dock). I’d say this was one instance when cost, service and environmental concerns are well-aligned.

A few things stike me about this. How do dd’s help the Royal Mail – if post that hard to stack?
Also..
>>The simple fact is that a greener Royal Mail is probably a smaller Royal Mail,….
yeah but what about a slower one too? I wonder if by increasing the del time, if improvements could be made?