I've done a lot of thinking on this, even crowdsourcing the coverage with people standing on sidewalks using their phones with a specific app, which collects similar data. Stand near the court house, politicians homes. However Flock tends to say that the people that contract them own the data to get around a lot of the liabilities, so they themselves "don't own the data". That is the hard part to emulate as a citizen who doesn't have same protections as a cop or city council.
I'm going off personal research though so I could be wrong.