Just imagine.... if in every TMA and airway over Europe, when climbing and descending aircraft were likely to get within 1000 ft, they built in an extra 1000 ft.... That would be like saying the standard vertical separation will now be 2000 ft..
Ah but how often do we experience a level off that is required to be made before the completion of the noise abatement procedure????
On the noise abatement we do not reduce the rate of climb to the ideal of 500 - 1000 ft per min with 1000ft to go for obvious reasons?
Therefore it should be obvious that if one is going to cross an aircraft 1000ft above an aircraft on a noise abatement departure, the rate of climb of the lower might be a bit on the high side for a simple 1000ft tcas (and STCA) free pass!
The "Maintain 3000ft" in city clearances is the most misunderstood example that I have come across. 50% of people I have operated with have said " as per the chart" and without being told otherwise would have climbed
at the appropriate point to 4000ft. That is not what ATC expected.
I think that it is amazing that so few problems have occurred!!
Based on what was going on in terms of non-standard clearance procedures, early level-offs, traffic descending on top it is pure luck that there were not far more problems.
As for "Cleared DVR xxx Maintain 3000ft". This is not taking a flight off the SID. It is simply repeating what is part of the SID.
I believe that if the SID is to be cancelled then a full clearance is to be provided i.e. something like "climb straight ahead to 1.5dme and then turn right to establish outbound on the LON xxx radial climb to 3000ft.
What has probably saved the day on many occasions is that the airport operator requires that all crews receive initial and regular refresher training before being authorised to operate into the airport and while a lot of time is spent on the arrival aspects, there is some time devoted to departures and the "local practice of wanting flights to maintain 3000ft "full stop" on the SID.