Method for installing a duct where an existing longitudinal member is present along a section over which the duct is to be installed. The longitudinal member is utilized as a guide for the duct. The tape-shaped element is closed around the longitudinal element through mating of complementary coupling devices longitudinally situated along the tape-shaped member so as to form a duct, with the coupled tape-shaped element then being propelled along the longitudinal element to extend the duct. The tape-shaped element can house an optical fiber cable. Through use of such a tape-shaped element, the duct, illustratively housing an optical fiber cable, can be formed around an existing (e.g. copper) telecommunications cable with any need to interrupt or remove that cable.