PURPOSE: To achieve the safety of traffic by burying magnetic materials in road surface to provide signs and tracking the same with a portable detector.

CONSTITUTION: Sintered magnetite plates of 100×800×15mm3 are buried at 100 mm intervals in the road surface of asphalt pavement, with the longer sides being made orthogonal to the advancing direction of the road, thereby proving signs. A detecting element 4 which comprises providing a gap 5 of 0.5mm on the side facing toward the sings to a coil containing Mn-Zn ferrite troidal core of high magnetic permeability is mounted to the end of a stick by means of a resin 7 and rubber 8. A self-excitation oscillator is made by this element as well as a ceramic condenser 6 (1.27μ) and IC and an LPF of less than 2K Hz is attached thereto. When the end of the stick contacts the magnetite, an oscillation sound of about 1K Hz is heared and as it goes apart, the sound becomes higher and becomes weak sound at above 2K Hz. This constitution enables the traffic safety of the blind person to be achieved.