As such, I believe there is an opportunity for an app that will allow citizens to help.
The app would run in the background on the person's smartphone, collecting time / location data (and potentially also mode of transport). When police officer records a crime, it is entered into a database, with the time and location. The app periodically (e.g. every hour) queries the database, and if there's a time / location match, then it notifies the user. The user can then think if they remember seeing / hearing anything, and have the choice to be put in contact with the police.
The key features of this system are:
- It's entirely opt-in for the users: they choose to download the app, and can choose to uninstall at any moment
- The potential witnesses' data does not leave their phone - there's no police state where the whereabouts of everyone is tracked
The service could monetise by charging the police forces a small administrative fee per crime uploaded (or per witness provided).
The benefit to the app users is that they can contribute to keeping their community safe (deterring crime through effective policing), with relatively little inconvenience, and no threat to their data.
The benefit to the police is an automated, high-reach way of finding witnesses.
The benefit to the criminals...? Well, hopefully they can be rehabilitated into being productive members of society.