The automatic adjustment mechanism consists of a primary activator which automatically displaces each support bar (10,11) at each free end of each element (8,9) of the cover film of a tunnel. A secondary activator tensions each element of the cover film during its opening and closing. The primary activator consists of two pulleys (15 & 16) of the same diameter, fixed to the driving shaft and located at each side of the point (12) of intersection of the elements of the arcs of film (3 & 4). Return pulleys (17 & 18), of the same diameter and rotating freely on the axis are located similarly. Two cables (19 & 20) link the support bars to the driving pulleys. The secondary activator consists of a spiral spring (21) at each end of each rolling bar (13 & 14).
