When Chapman’s car triggered that alert on the evening of October 26, it took the police just 20 minutes to find and stop him. But, as a later investigation discovered, it was not the first warning that had been issued. In fact, a total of 16 automatic number plate recognition (ANPR) alerts had been put out over the previous three days —including four on the day he was arrested. Three police forces spotted Peter Chapman. Between them they cover a combined area of more than 10,000 square km.

The combined area covered by Cleveland, Durham, and North Yorkshire police forces is over 10,000 square kilometers. It is policed by close to 5,000 officers and home to almost two million people —similar in size to Houston, Texas— but spread across an area 10 times greater.

The report for Chapman’s vehicle said the driver was “to be immediately stopped”, but it was only graded as medium priority. In truth, the alerts were just a tiny handful of those that tumble onto police computers in a never-ending avalanche of data: in Cleveland alone, roadside cameras generate around 2,500 alerts every day.

Officers were sent to find his car six times, but for four days attempts had proved futile. After all, knowing where a vehicle had been 10 minutes earlier is not necessarily enough to find it on Britain’s crowded road network.

In the space of three days, 16 alerts were generated by Chapman’s car. Each one urged police to arrest the driver. […]