Well, I played about with this sort of stuff a loooooooooooong time ago, so I've probably forgotten a lot of it.
I should have written it all down.
From memory:
AI infantry will cross bridges nearly every time, there are occasions when they refuse or have some difficulty though.
AI cars and trucks seem to have no difficulty at all.
AI armour almost always refuse to cross bridges unless they are turned out or in "safe" or "careless" modes.
If any of the above meet something else on the bridge nothing I have said above applies.
In other words nothing works well any more.
Planck