bookworm, the example that prompted this thread is about landing clearances - and the issue of a landing clearance is largely determined by the risk of a collision on the runway. However, I have specifically talked only of principles and, as principles, the can be applied generally.
At no point have I intended to suggest that a controller can take on the decision-making related to many aspects of the safety of the aircraft but, on the contrary, I have tried to balance my comments with the non-normal situation in which a pilot can quite correctly act in a different manner than would be expected in response to an ATC instruction. I did not consider risk management issues because the thread was about the 'legality' of landing when a landing clearance had been cancelled (although for various reasons I could disappear along that particular tangent for hours if you really want!) .
As you rightly point out, neither pilot or controller will have the full picture but I stand by my statement that in normal operations, in the context of whether or not a landing clearance can be issued, the controller is likely to have the better understanding of the determining factors.
I agree that the application of 'separation for separation's sake' is not appropriate or desirable but I would argue that in many situations that is the primary role of a controller so you can hardly blame him or her for treating it as important. Of course, there are times when traffic is over-controlled and I would support you completely if you are suggesting that standard separation is sometimes unnecessarily applied when far less distance between aircraft would far better facilitate the safe and orderly and expeditious flow of air traffic.
Finally, having said all that about separation, I would also respectfully suggest that the role of an aerodrome controller is about far more than simply avoiding collisions.
172driver, you are quite correct that before my earlier post I only skimmed through the other thread. As you advocated, I have now read the thread carefully. I'm sorry to disappoint you but I find nothing that inclines me to change my view.
Your statement that 'Once cleared, the rwy is yours, period.' is incorrect. I will reiterate my concerns that making the statement can cause much harm by planting a fundamentally incorrect idea into minds that have not previously questioned the point.
In your later post on the other thread you ask 'Some of the ATCOs here care to comment ?'. I have done just that. And I'll repeat it once again - your assertion that ''Once cleared, the rwy is yours' is wrong. The way in which landing clearances are issued in different parts of the world do differ (particularly in respect of the application of ICAO SARPs on anticipated landing clearance), however, I would be interested to know what rules you believe support your assertion (and just to play fair, I'm happy to enter debate about international or UK rules).