Fuji
There are other reasons you will not use airways. I often have a descision to make with the operation of a jet on short legs where you have a choice of filing at something like FL160 or staying away from CAS and accepting a low level.
That choice is do you file IFR and airways and take 45 minutes on the trip at FL160 or take the lower level and take 25 minutes because its far shorter.
Sometimes you take the shorter route using RIS or RAS and fly fuel flows accepting the reduced speeds and with a jet far reduced costs?
Ie on certain routes especially short ones for a jet the airways routing is far longer ie Biggin to Bournemouth.
I am sure some P**t will attempt to jump on me
Pace