Recessed speed reducing system for street traffic, comprising a speed bump, composed of a plurality of modules, recessed into a corresponding housing near the carriageway and associated with suitable actuators, designed to be suddenly brought from rest position, in which it is completely contained inside the said housing, to working position, in which it partially protrudes with respect to the surface of the carriageway, when one or more remote speed detectors that cooperate with it detect an approaching vehicle travelling at excessive speed, and designed to be suddenly brought back to rest position when one or more sensors (106, 107) that cooperate with it detect that the vehicle has completely passed over the said bump, characterised in that each module of the speed bump comprises: - a basically U-shaped frame formed of a horizontal wall (1 a) with two identical opposite wings (1 b) protruding from the sides, - a platform transversally fixed in central position on the upper side of the said horizontal wall (1 a) of the frame in intermediate position between the two vertical wings, - an actuator fixed above the platform designed to actuate a stem able to make alternate vertical travels, - a grated head fixed at the upper end of the stem and composed of a rectangular plate with horizontal direction, above which some transversal partitions in edgeways position with upper convex curved section are mounted and regularly spaced, - a horizontal grid supported above the said grated head by uprights fixed to the platform; it being provided that each slot exactly receives one of the transversal partitions of the grated head in such a way that, during the alternate vertical travels of the said grated head, each transversal partition is brought from a lower stop position, in which its top convex section is inserted inside the corresponding slot to an upper stop position, in which it considerably emerges above the said slot, - an elastically stretchable strip that exactly covers the upper surface of the said grid, being fixed at the ends on the transversal borders of the grid. - a basically U-shaped frame formed of a horizontal wall (1 a) with two identical opposite wings (1 b) protruding from the sides, - a platform transversally fixed in central position on the upper side of the said horizontal wall (1 a) of the frame in intermediate position between the two vertical wings, - an actuator fixed above the platform designed to actuate a stem able to make alternate vertical travels, - a grated head fixed at the upper end of the stem and composed of a rectangular plate with horizontal direction, above which some transversal partitions in edgeways position with upper convex curved section are mounted and regularly spaced, - a horizontal grid supported above the said grated head by uprights fixed to the platform; it being provided that each slot exactly receives one of the transversal partitions of the grated head in such a way that, during the alternate vertical travels of the said grated head, each transversal partition is brought from a lower stop position, in which its top convex section is inserted inside the corresponding slot to an upper stop position, in which it considerably emerges above the said slot, - an elastically stretchable strip that exactly covers the upper surface of the said grid, being fixed at the ends on the transversal borders of the grid.