If you build a surveillance system, the police will come.
There’s a cautionary tale for people who travel by public transit in the latest report on collaboration with Canadian police by the Toronto regional transit agency, Metrolinx.
If you build a system that keeps personally-indentifiable logs of when and where people go, eventually the police will want to exploit those logs for their own purposes — regardless of the original purpose for which the data was collected, or the policies that, at the time the data was collected, purported to restrict how it would later be used.