I disagree with some of the posted statements.
New compact DV units or "GoPros" are out there. Retailer Brookstone even has a small drone citizens can run with a smartphone/iPhone. I played with it in Stamford CT,
Cops in 2013 have camera systems in sunglasses, Tasers, tactical vests, weapons, vehicles, etc.
Growing up, I often heard: "if it's not written down then it didn't happen".
Remember too that what the US criminal justice system is based on is not what the truth is but what you can prove
I'd add that I don't suggest anyone hold a weapon & a camera phone, but be ready & able to protect the crime scene/evidence or gather information.
I'd also keep your legal & civil rights in mind at the scene,
Not to stray off topic, but in my area a car lot mgr was shot & killed in a "road rage" event. He was test-driving a customer vehicle for a trade-in and a armed citizen shot him with what looked like a Colt 1911a1 Defender .45acp.
I'm sure the car dealer CEO/legal affairs will now have employees drive in pairs or have DV cameras/mini cam systems with them.