It is possible to get around the gate on the south side to get onto the bridge before getting the necessary cards. Doing so allows you to encounter the purple mist and the suicidal man, and then the large monster that takes out part of the bridge. Running away fast/far enough brings up the "End of Part One" message, but does not end player input (this may be intentional/normal; this was my first playthrough).
Crossing the bridge after this event (but before opening the gates) leads to an odd glitchy state. Standing on the far eastern edge of the bridge, the last place the player can be without suffering the Wasteland 2 debuff, the bridge will occasionally completely disappear, so the player appears to be standing on an ordinary coast tile. Moving up/down re-places the character on the bridge. Very unusual!
After the monster has left the hole in the bridge, but before the player has constructed the wooden replacement to cross, it is possible to fall into the pit below, where the player becomes trapped and unable to escape--no choice but to freeze to death, as the Wasteland 2 debuff is present there. There are a number of geometry concerns in this area. The player can fall into the bed of the truck at a steep angle, and get trapped there (though it is possible to escape by throwing wood/ore/inventory items to 'fill in' the hole--this also allows the player to step OVER the cabin of the truck itself and get further east on the bridge, leading to the concern above)... Stepping gently onto the western edge of the ghost bridge (the illusion of the bridge before it is built) allows the player to fall onto a shelf just below the base bridge--a small cave beneath. Unfortunately, the player is completely trapped here and again cannot escape (though I threw about 200 wood under the bridge, which is still visible when I hover the cursor around that part of the ground).
All of these collision concerns go away after the player constructs the replacement bridge--a huge invisible wall prevents player from getting anywhere near the tilted truck and/or from falling into the pit off the sides of the replacement bridge. Items that were lost/trapped underneath the bridge are still technically accessible by going down to the shoreline and walking UNDER the bridge, but again, if a player moves too far west into the clutter beneath the bridge, they risk becoming trapped yet again.
My instinct is to either recommend the geometry is not steep enough to trap players, such that there is always a path to escape from the debris beneath the bridge, allowing the player to end up on the shore directly to the north or south of the bridge... otherwise, take a note from the post-replacement bridge state and prevent the player from advancing into the pit at all until the replacement bridge is constructed.
As a side note, because I did these events out of order, I worry my game is stuck in some sort of limbo--exploring the area across the bridge has not led to any events... just a lot of Wasteland 2 debuff and backtracking to warm up.