From the manual (block properties):

INVISIBLE: invisible obstacles. Note that invisible blocks are also not detected by trace() instructions. If a block should be invisible, but detected by trace() , use the None texture flag instead.

I have confirmed that invisible blocks are just trespassed with c_move.
On the other hand, the suggestion of using the None texture is no good.

I am not sure if I want to convert all my ramps into map entities! Isn't that a lot of work? There should be an easier way.