Systemwide EXG installations since 2006 introduced a weakness into otherwise secure AFC systems. Gates were originally only unlocked via booths' buzzers or employees’ keys. After London Underground’s 2005 terrorism attacks, fire codes required “panic bars”, allowing each gate to be opened from the paid side, expediting emergency evacuation. While a loud, piercing, and warbling alarm sounds whenever EXGs are opened, general public took to using gates for exiting (substantially reducing queues), especially at unstaffed locations. It happened so often that in January 2015, the MTA silenced all exit alarms.