|The following is based on Fallout Shelter or Fallout Shelter Online and some details might contradict canon.|
Rushing is a mechanic found in Fallout Shelter. It can be done at anytime to speed up the production of a room. However, there is always a chance to fail, which will trigger an incident. An initial rush's odds of success are derived from the assigned dwellers' SPECIAL stats which affects that specific room and their luck. For example, the odds of success for rushing a power generator or a nuclear reactor would be affected by the assigned dwellers' Strength. After attempting a rush the room's production timer will reset, regardless if the rush passed or failed.
A successful rush will gradually increase the assigned dwellers' Happiness by 10%, while a failed rush will decrease it by 10% within 1 minute- unless the dwellers are reassigned before the 10% are reached. Note that pregnant dwellers will not respond to the disaster, and will instead flee in panic. They will automatically return when the disaster is resolved. If you send dwellers to respond to the disaster, they will return to their jobs automatically, and not stay in the room with the disaster.
The chance of a rush failing will increase after each rush attempt by 10% each time, and will not reset until the game has been exited or a moderate length of time has been passed. Before rushing, be sure to equip dwellers with weapons in case of an incident occurring. Only production rooms may be rushed, training cannot be rushed and neither can the storage room, the living quarters, the vault door or any elevator.
It was possible to reach 0% incident risk when rushing with dwellers with very high SPECIAL. However, a new update makes it impossible to reach 0% incident risk regardless of high SPECIAL.
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