I've seen some better-than-bad performances from AI in bridge battles. In my battles AI has occasionally sent a single unit to cross over a ford while most try to pass through the bridge. At one point AI sent ashigaru unit walking that way, I sent my No-Dachi samurai to counter them, they turned back and so did I, then they turned around again and so did I, this continued for a while...

AI really should consider where to cross. In all my bridge battles, I would have been completely caught off guard if AI had sent all units to a ford or even split troops equally.