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Just because the reader knows the reason for the Bridging Conflict doesn't mean the characters will be able to overcome it. In your example, even if the reader knows all the extra security measure...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/17815 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/17815 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Just because the reader knows the _reason_ for the Bridging Conflict doesn't mean the characters will be able to overcome it. In your example, even if the reader knows all the extra security measures are because of the Zombie Apocalypse, that doesn't mean that the zombies won't break through the security measures. In fact, it lends some extra tension. The reader may not wonder why the city is gated, but may now wonder if the zombies will break through the gate before the heroine has a chance to create the cure.