http://www.bonnaroo.com/Have you search the web for it?
Done a news search?
Find out the town where the event is located do another search for that towns news, find that news papers web site search for past articles ready any replies from the users to get a feel for the event.
Locate the law enforcement that covers the are where the event is located send them a email tell them your concerns ask how many were arrest were connected to this event in the past.
Really have spare time locate the hospitals in the area contact them ask how many people ended up in their hospital during one of the events.
Do the same for the area where you plan on getting a motel.
I know nothing about the event but I bet its not going to be much different than any other similar event any place in the world all the dangers will probably be about the same.