Abstract: Coded binary signals are coupled to an automotive vehicle by a magnetic signpost embedded in a traversed roadway lane. The signpost includes polarity coded magnetic pole faces having vertically directed flux lines at differing spaced longitudinal regions along the lane. Pole faces are arranged so a flux null is between adjacent longitudinal regions. Differing signals are coupled to vehicles going in opposite directions in adjacent lanes by unambiguously coding the signpost in opposite directions and arranging the pole faces so that magnetic flux continuously extends across a majority of both lanes. A detector on a vehicle includes a magnetic field concentrator including a pair of vertically extending and aligned low reluctance magnetic pole pieces having an air gap between them, in which a Hall plate is positioned. The pole piece closest to the road has shorter length than the pole piece remote from the roadway.