A crash bar is a type of door opening mechanism which allows users to open a door by pushing a bar. While originally conceived as a way to prevent crowd crushing in an emergency, crash bars are now used as the primary door opening mechanism in many commercial buildings.

Crowd collapses and crowd crushes are catastrophic incidents that can occur when a body of people becomes dangerously overcrowded. When numbers are up to about five people per square meter, the environment may feel cramped but manageable; when numbers reach between eight and ten people per square meter, individuals become pressed against each other and may be swept along against their will by the motion of the crowd. Under these conditions, the crowd may undergo a progressive collapse where the pressure pushes people off their feet, resulting in people being trampled or crushed by the weight of other people falling on top of them. At even higher densities, the pressure on each individual can cause them to be crushed or asphyxiated while still upright.