My final words on the subject
We are not talking in airline nor aircraft terms, we are talking about a darn bus trekking the roads of South Wales and Severnside.
From Swansea it will enter and trek along the M4, rather than continue 'DIRECT' it shall deviate probably exiting the M4 at J33 to then hit the wonderful delights of the Cardiff City traffic.
Upon exiting Cardiff it shall probably, once it has shrugged off the Cardiff City traffic, exit Cardiff along the A48/A48(M) rejoining the M4 at J28, no sooner will the bus have picked up speed it shall be exiting the M4 again for a repeat performance of the Cardiff situ but, this time, in Newport.
And thereafter it shall head towards the Severn X-ing where perhaps an 'English' commentator may take up the scenic tour of Bristol and the surrounds.
Just as an example, were it to continue along the M4 past Cardiff J33 to J28 might take 10 minutes, routing thru Cardiff for pick-up/drop-off will probably take 45 minutes to an hour and stopping in Newport, never mind the stops the other side of the Severn, will lengthen the journey time even further.
About the only thing that will last long about this service are the journey times.