In fortress design, it is desirable to put bridges on your caravan entrance so as to stop thieves from using it. Being three-wide with no doors, it is difficult to defend otherwise.
But DF's mechanics present a problem, in that caravans arrive unexpectedly and do their pathfinding instantly. The player is forced to lower the bridges at the start of each season (except winter), leaving his defenses open longer than ideal.
Worse, the caravans don't always show up -- in my last fort the humans skipped me in the second year, but showed up in the third. So I had my back door open uselessly in year two summer, but got bypassed in year three because I didn't expect them and didn't lower the bridge.
I have one idea to fix the problem -- allow the player to set a "Caravan Waypoint". When a caravan arrives and cannot immediately path to the depot, it will instead path to the waypoint and then wait for a path from there to a TD to open. Likewise, on exit the caravan will head to the waypoint if there is no immediate route to the map edge.
Thus, the player can set his CW to the outer lip of his normally raised depot bridge, and have security without the risk of wagon bypass. The player can even create an "airlock" of bridges, although the outer one will still need to be open by default.