In reply to nicjbuk:
> Provided you are efficient and careful, and keep looking up the coulior as you cross, it is no real problem. (...) I reckon the crossing is about 50M length of exposure?
Yes, that's exactly what I always used to think. But I was wrong. Sure, I have crossed it numerous times when it's seemed totally benign - as it appeared to you. One day it wasn't and I was hit by a large spinning dinner plate across the back of my calf which cut deep into the muscle - this despite being careful, cautious, waiting till I judged the time was right etc. I saw it coming just at the last moment when I'd just started crossing but was completely powerless to avoid it - all I could do was duck down. I was in descent and it just about doubled my time down to Nid d'Aigle. After the adrenalin had gone, I couldn't walk for a week. I was incredibly lucky. If you think how close the back of your leg is to your head... especially if you duck!
If you're not convinced then it seems like time to have a look at this again:
You are at risk for a long way above the point where the traverse meets the long easy rocky rib on the far side. Depending on the size and amount and provenance of the rockfall, I'd say anything from a third to half height, though obviously less so than in the couloir itself.
But it's the same with all things alpine - if nothing happens to you, then you've a tendance to shrug it off. For instance, I've never seen a serac fall actually happen on the Tacul, but statistics tell me that sometimes it's a lottery. As is the Gouter couloir. And this is why I stopped guiding Mont Blanc.