Two Donkeys is quite correct.
These corridors of airspace are designed to keep you out of the way of pilots who are not flying their aircraft visually. They are simply following a Director and a radar map. Actually, this kind of operations happens all over France, and the areas you see on your maps. are just the busiest corridors which the State has chosen to protect with laws.
The best advice in France is to climb above 1500 feet ASFC as soon as possible, and to stay there. Do not get too 'hung up' about the precise definition of 1500 ASFC. The intention is that you should be high enough above the ground to let a couple of us go underneath you very fast.
Ah memories...