The UK Highways Agency (HA) has a specific interest in understanding the actual journeys taken by drivers using the London orbital motorway (M25) in order to assist with planning and management of strategic routes. It is thought that the majority of traffic on Public Service Agreement (PSA) Routes, rather than travelling end to end is joining and leaving mid-route. We provide a tracking algorithm to detect journeys on the routes using data from the existing automated number plate recognition (ANPR) network. This network was designed for other purposes and in particular only covers subsets of lanes at sites, requiring inference of traffic behaviour in the non-covered lanes. Furthermore, its cameras translate plate numbers into nonunique hash values which give rise to many spurious matches. A full-lane calibration study was performed, and fused with induction-loop flow data to obtain parameters for such inferences
