Thanks a lot for your comments!
FvM - Yes you are correct on ILD being the most common sensors in traffic managment. Even in Auckland, IDL are used at almost all traffic lights and also on the motorway. Despite being such an old technology, IDL still does not provide accurate data due to many factors. IDL is sensitive to the speed of the approaching vehicle, height of the conductor (vehicle) , weather, installation procedure used, detector technology used etc . I've been reading up on ILD for the past 1 month now and so far I haven't come across a hardcore formula saying that at this speed, this height, this many turns in the loop -THIS would be the change in inductance. It is very difficult to parametrize the ILD output. The problem increases even more when the output is used by a traffic control program (ie SCATS).