1. On sections where the course to be followed is prescribed, that course shall be marked by the mandatory signs B.1, B.2, B.3 or B.4 (annex 7). The end of the section may be indicated by the informative sign E.11 (annex 7).
2. On such a section, vessels proceeding upstream shall in no case impede the progress of those proceeding downstream; in particular, on approaching the mandatory sign B.4, they shall if necessary reduce speed or even stop to allow vessels proceeding downstream to manoeuvre.