Since Fristgrave is undergoing a massive thaw, I picture the rivers being flooded and rushing with melt runoff. So, in my mind the current and debris would be very dangerous.
Bridges are notorious killing fields. They could have plenty of cover to make for a safer crossing, or you could go the other extreme and make them a "troll bridge" with an opportunistic troll or 2 making use of them for an easy hunting snare.