Speaking of Music Festival crowds, having worked at Love Parade a few years back where there were fatalities due to lack of crowd controls and at Tomorrowland last year, I was really impressed with the Sendrato bracelet that Tomorrowland use (http://sendrato.com/ ) which can be potentially used to determine densities of crowds in a particular area, and allow controls to be put in place before problems arise.
Basically, exits were blocked off on both ends and the crowd was large enough that people trapped in the middle were crushed by enough force to led to suffocation. People attempting to move caused compression waves to ripple through the crowd which injured hundreds.
