The barrier has upper and lower side sills joined together by fixed or movable posts. A control rod has a set of notches, where a sliding pin is inserted in each notch. The sliding pin is guided in a fork of a pivoting lever. A locking notch (1) of the pivoting lever is formed on a cylindrical sleeve, and traversed freely by a pivoting rod (4) whose longitudinal axis is different from axis of a sliding sleeve. The sleeve has two radial extensions (7, 8) and axial immobilization units that ensure axial immobilization with respect to a bearing (9) integrated with the upper side sill.
