Trunk roads use a different technology to cater for the different traffic patterns experienced on such roads. Traffic turns off at junctions and may stop in lay-bys, at shopping centres etc, so simple speed measurement would not generate quality data. Passive Target Flow Measurement (PTFM) uses number plate recognition technology to ”grab” the four centre digits of a vehicle number plate. This is turned into a four figure electronic ”tag” on site – no number plate data is retained.
As the vehicle proceeds along the road and passes the next site(s), average journey times between sites are calculated and sophisticated computer programmes establish the speed of the traffic over those ”links”. In a seamless process, traffic speed on that particular section of road is then delivered to the traffic information product.