OK, time to revive this one a little with the follow-up question. Please bear in mind in what follows that I'm aware that the information delivered in a radar information/advisory service may be prescribed by regulation (in particular MATS Pt 1 Section 1 Chapter 5 15.1). If I question the status quo, I am not for a moment suggesting that ATC is doing it wrong, but rather that the system itself is not optimal.
The problem with the traffic info as passed is that the movement information (e.g. "crossing left to right") is of no use to me as a pilot.
Any aircraft on a converging track will have no relative motion with respect to my aircraft (or more correctly, the relative motion will radial -- directly towards my aircraft -- with no tangential component). If the relative motion is towards the rear of my aircraft (e.g. 11 o'clock to 10.30 or 2.30 to 3 o'clock) then it will pass behind. If the relative motion is towards the front of my aircraft then it will pass in front.
The action that I take to increase separation depends on which of those situations is the case. If it's going to pass in front, I'll turn towards it. If it's going to pass behind, I'll turn away from it. If it's coming straight for me -- well it doesn't really matter which
way I turn, as long as I
turn.
"Crossing left to right", or whatever, gives me no useful information. If the traffic is in my 10 to 11 o'clock then it's bound to be crossing left to right -- otherwise it's going away from me or passing harmlessly down my port side and there's little point in calling it.
What I need to know the answer to is "which way do I turn to increase separation?". Of course I can
guess which way to turn, but that will as often decrease the separation as increase it. I'm not asking for 5 miles here, just an indication of which turn is going to make life better.
In an ideal world, you'll warn me if the other aircraft continues to be a threat, and I'll see it before the crunch. The only problem with that is that it's the aircraft with no tangential relative motion that are the hardest to see. So we need to work together to make sure that safety is maintained even if the world is less than ideal.
Two questions:
Is it usually apparent which of the three cases ("passing in front", "passing behind", or "coming straight at you") is happening? (I understand that aircraft don't always travel in straight lines, but most of the time they do!)
And if so, what phraseology should be used to help the pilot distinguish, in other words to help me know which way to turn?