trevHCS:
DD trailers are not better for the environment as even empty they take a lot more pulling thus more fuel etc.
DDs on pallet system work are rarely empty as they take full loads to and from the hub; this is the very essence of a pallet system. You can get 26 pallets on the bed of a standard trailer; some can be doubled up, but a top deck gives space for 18/20 more. Fuel consumption, fully laden is not much different to a standard artic.