As far as I know it is legal to film public officials in the course of their jobs so long as you do not interfere. I have seen quite a few cases mentioned, but cannot vouch for the legality in every state. of course, you might want to have a second person filming surreptitiously in case a LEO decides that "interfering" includes standing across the street filming. Also, it is not recommended that one point anything at police in a manner which might be deemed threatening ... even a cell phone. The suggestion is the keep the camera or phone in front of the body between chest and navel and not extended.
I have had LEOs object to me taking photos at events ... best thing to do is decide whether you want all your gear smashed and to go to jail or not. if you have a lot of financial and legal backing, do whatever. if you figure it would be better to keep covering the event from a different location, do that. But in an accident situation, i cannot imagine a LEO would object, unless the LEO was grossly at fault (like that trooper that hit a cyclist at a "T" intersection, and was videoed.) If i were questioning whether I might lose my cameras, I would take the ticket and bring the video evidence to court. otherwise, I would certainly inform the officers at the scene that I had the incident on video.
All that is hypothetical. I simply don't go around places where bad things happen.