act of God?
Time to look at this from the social science point of view.
Interesting that all Europeans followed the GB initiative on the basis of their evidence. The trigger was safety, economic problems, medical need or inconvenience were not built into the model. None of the authorities had a plan to end the ban before they started it. Governments are left to balance the conflicting interests, and understandably cautious.
The approach could the same as banning road traffic on rainy days, as there is a definite safety risk. The main difference in approach is that flying is completly regulated, and driving, while highly regulated, is also in the hands of drivers and insurance companies. One problem is the grey area of ash levels between safe and unsafe (see chemicals regulation). Another is finding safe ways round the ash.
A further problem is finding workarounds to a ban in a regulated area. France has reduced road speed limits on rainy days, Moto-GP has wet and dry race rules. Denmark today permits flights above 35000 ft but not below - illogical for an interntional solution, but helpful in some cases.
One conclusion might be to temporarily deregulate, leaving interested parties to find solutions to their problems. If necessary, the European airlines could force this through, since they have been made responsible for acts of God.
Except of course, that this a catastrophe of committees, and very much an act of Man.