An exit device is a type of door hardware which allows doors to remain locked from the outside. People inside the building, however, can still exit quickly without having to unlock the door. The mechanism actually makes a hands-free exit possible. In fact, the nickname "crash bar" is derived from the ability to open the door by simply applying full-body force to it ("crashing" into it.)
These doors cannot be opened from the outside when locked. In the event of an emergency, such as a fire, those inside the building can use these doors as emergency exits by activating the crash bar on the inside of the door. The term "panic bar" came to be applied to exit devices because they can be opened quickly in an emergency. Panic bars are also advantageous because people with limited mobility (i.e. those in wheelchairs) and even small children can activate them.
