Ferris,
In the old days of VOR-VOR-NDB-MARKER navigation your comments regarding flights suddenly veering off to some unknown place held some truth - you tracked beacon to beacon and every beacon was identified prior to use and the system ensured that you could never receive two VORs/NDBs with the same ident even up to 50,000ft.
The same can not be said today in the world of FMS and GPS databases...........direct to say DUB could take the aircraft towards a place near you or direct to Ireland...........Thus an aircraft in your area of the world passing a intersection and then the FMS being told to go direct to DUB..........the tired pilot misses the fact that there are two DUBs in the database, the aircraft may suddenly head 30deg right of the expected course............just when you are on the phone and not watching the screen..........before you know it.........that flight is getting toooooooo close to another climbing throught it'slevel and you could have prevented it.
You sould have a read of Attachment A and B to Annex 11.
In annex A, the spacing for parallel routes defined by VORs is basically 18nm with a reduction to 16.5nm for traffic going in the same direction on both.
In annex B, the separation for traffic on RNP5 routes is 10 to 15nm.
There is more involved. However, taking the lowest of the above figures - traffic has to be atleast 10nm parallel and radar monitored to continue on own nav at the same level............try that round Europe and the movement rate would halve or worse.
As a pilot being on a vector or locked on a heading is no big problem........unless I am being taken all over the place - which route direct and continue the headig when established is definitely not.
Regards,
DFC