As a frequent rover/mobile op in various contests, I have lots of
stories about "interactions" with law enforcement officials.
Most of them were unpleasant. Now I write a letter to the appropriate
organization a couple of months before the contest humbly requesting
permission to operate and giving whatever details I can.
Since then, I have been welcomed with open arms, and have had
many opportunities to give demonstrations to our various protective
forces...(which can be a bit annoying during a contest...). I have
never been turned down when I have written a letter!