This feed gets an update every time a vehicle is detected leaving Adastral Park via the main gate, currently implemented using ANPR (Automatic Number Plate Recognition). Usually the update value (which is a count of the number of vehicles just detected leaving) will be 1, but it is possible that if two vehicles are detected at the same time, it could be more than 1.

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Additional terms: http://datahub.rp.bt.com/