The routes are used by training flights that frequently operate at speeds of up to 500 knots using terrain-following radar, radio altimeter and flight director equipment. Such aircraft are obviously unable to obey the normal rules of the air, and the corridors shown on the maps are designed to be wide and tall to avoid confict.
Most of the time, this means that the military aircraft concerned will be around 250-500 feet above the ground, leaving plenty of space in the tops of the corridors.
The intent is that GA and other flights comply with the corridors by flying at 1500 feet or better above the surrounding terrain. You are expected to use your intelligence when it comes to small valleys and bridges, rather than to seek too literal a translation of ASFC.
It is worth noting that high speed low-level military flights may take place at any point within French airspace and the French AIP urges pilots to ensure that they spend as little time below 1500 feet as possible, regardless of the presence or otherwise of such a defined corridor.