Abstract: A system and method for transferring historic and instantaneous diagnostic data relating to performance of an inductive loop vehicle detector 10 is disclosed and claimed. The aforementioned date is gathered over a period of time, updated and stored in a detector unit 14 forming part of the system. The stored data is then used repeatedly to modulate visible light emitted by a light emitting device 22 on the unit 14 at such a rate that the data would not be discernible to a human. The data is received by a portable data reading device 24 which is brought into optical communication with the device 22.