>>>why does anone need that data collecting Chris?
I'm not 100% sure, to be honest!
However I assume that it's part of a wider study, monitoring traffic movements around parts of the town. The company I'm working for has got automatic traffic counters set up around the area. (They're those rubber tubes you see stretched across the roads). They've also got lots of cameras in place but (I guess) they need us guys to provide a sample subset of the data [by just collecting numbers from white & silver cars] to see how the traffic is flowing.
So if I report that vehicle 1CAU passed me on a side road at 1037 - we only record the last 4 digits - and the guy around the corner (to the left) on the main road sees it at 1038, we'll know that the vehicle turned left out of the road I'm monitoring. It could then be picked up again at other points in the area, indicating the path it followed.
At the end of the day we'll have loads of data for Hertfordshire County Council to analyse and then (most likely) ignore!