On the right is the plot of a real time measurement of the magnetometer for a vehicle moving from x- to x+ with the sensor mote placed on road side, sampling at 64Hz for each axis.

When the vehicle is approaching, the magnetic field line would be distorted towards the vehicle, which is along the negative x-axis of the sensor in this case. So, a decrease in the x-axis measurement is resulted. When the vehicle is in-line with the sensor, no distortion would be caused on the x-axis, which brings the measurement back to the base level. Similarly, when the vehicle is moving away from the sensor towards the position x-axis, an increase in the x-axis measurement is resulted.