Crowd protection device

Kind of a riff on the rave idea below, I was thinking about how people get killed when crowds press forward without regard to fallen individuals and wondered if technology could help by signaling when someone has fallen down. For example, imagine a watch that can detect when the wearer removes it or falls down (e.g. using an accelerometer). Suppose this watch also has the ability to notify similar devices in the area when one of these two events occurs, and those watches continue to spread the message outward. Then if everyone in a crowd was equipped with such a device and someone fell, the watches would alert people to stop moving forward (e.g. by vibrating and playing an audio message). Because the message propagates locally, and entire densely packed crowd would be alerted as the message spread from watch to watch. This might work to control low-speed crushes occurring at organized events like the Haaj (spelling?), sporting events, etc. It probably wouldn't make much difference in case of a fire.