I certainly wouldn't want my bikes on a towbar due to the rubbish standard of motorway driving that goes on.
Nobeerinthefridge's comment about bike should not fall off – well mine did due to the Thule foot shearing (estate car) but it did stay attached to the Thule bike carrier. Don't get me started on Thule. 🙄
The comment from the chap about not being able to reach a roof mounted carrier – my ex made a wooden step for me, very sturdy.
I just put mine in the boot or wheels off and placed in a bike bag for holiday duties.