I leaned a trick from some Captains that worked on the N.Y. State Barge Canal System years ago. Now this only works if you can get eye level with the highest point on your rig. When looking towards the bridge (eye level with the highest point on your rig) if you can see the rivets or the beam on the far side of the bridge you will fit as it means that you are looking up at it, now you may not clear by much but you will clear.
Now I realize this method will not work while running down the road but it will help if you get stuck and stopped at a low bridge.
