Wonderful, wonderful visual explainer (via How a Car Engine Works - Animagraffs)
When the police used ANPR data to study Khan and Uddin’s movements, they uncovered another surprise: the duo hadn’t travelled alone. Officers dug into the national database using a technique called “convoy analysis.” First, every record of Khan and Uddin’s car trip was recalled from the system. Then another set of plate numbers was generated: those of every car that had passed by those same cameras within a few seconds or minutes of the pair. By comparing this set with those at the next camera site, and the next, and the next, the police identified a second car that had travelled in convoy with them from Birmingham to Dewsbury. Within 48 hours, police arrested not only Khan and Uddin, but four further members of the group from the second car. All six men later plead guilty to preparing an act of terrorism, and were sentenced to a total of 111 years in prison.