Abstract: Two orientated magnetic field generating means such as permanent magnets are embedded in a roadway at some distance from each other before a road-sign, said magnets extending on the whole width of the roadway. When a vehicle equipped with a detector crosses said magnets, signals of pseudo-sinusoidal shape are generated. Said signals are applied to elements converting them to calibrated pulses corresponding to the two alternations of the signals. The order of said ulses is checked to validate only pulses appearing in a predetermined order, thus eliminating direction ambiguity. The time separating two pulses is also checked to validate only pairs of pulses comprised in a predetermined time period, thus eliminating interferences from outside. The pairs of pulses are transmitted to at least one time window circuit which is adjusted to a predetermined period of time. Only pairs of pulses appearing within said predetermined period are validated.