Bridge Over OB (Multiple Playing Surfaces and Verticality)

Question: My throw landed on a bridge that spans an OB creek. The TD has not said anything about playing from the bridge. Do I play from the bridge, or is my disc OB since it's above the creek? What if I'm on the bridge but over land? Does it matter if the bridge is more than two meters above the ground below?

Response The answers to these questions revolve around the definition of OB. In the glossary section of the rules, it states that the OB line "extends a vertical plane upward and downward". Where does that plane end? The rules do not address that directly.

There seem to be two reasonable choices:
A: The vertical plane extends indefinitely up and down.
B: The vertical plane ends when it reaches another playing surface.

Option A requires less interpretation, and option B makes more sense intuitively. The Rules Committee has discussed the issue and has decided that option B is preferable.

Although the term "playing surface" is not defined in the rules, it is used frequently and it is unlikely to be a source of confusion. Something is either a playing surface or an object on the course. A bridge, though man-made, is intended for foot traffic and clearly qualifies as a playing surface. Since it is not an object on the course, the two-meter rules does not come into play.

The IB/OB status of a playing surface is not affected by the OB status of another playing surface above or below it. OB applies only to the playing surface that contains it. Otherwise, a number of non-intuitive rulings result:

In the bridge example, the part of the bridge that is above the OB creek would be OB. A perfectly playable lie on the bridge could be OB, a foot away from a lie that is IB, when there is no direct reason for it to be OB. Players will have difficulty extrapolating where the OB part of the bridge is, especially if the OB line below is uneven (if it follows the creek's edge). Even if the TD uses paint or string to mark OB on the bridge, those lines will see a lot of foot traffic and may not last.

At least one course has an OB culvert that runs under and opens into a fairway. If the vertical plane of the OB line extends indefinitely, then there is a strip of OB on the fairway over the culvert. If an OB creek undercuts a bank, then the top of the bank is OB even if it is obviously playable. Someone would have to determine how far the creek undercuts the bank to figure out just where the OB line on the bank is. There is an overpass with a street high above a section of the course. The street, of course, is OB. If the plane extends downward, then a street-wide chunk of the course below is also OB.

If you interpret the vertical plane to end when it reaches another playing surface, you get much more intuitive rulings in the above scenarios. The bridge is IB, the fairway above the culvert is IB, the bank that overhangs the creek is IB, and the ground below the street overpass is IB. All of the playing surfaces above are easily distinguished from those above or below which contain OB.

Conclusion: You play a disc on a bridge as you would play it anywhere else on the course. Assuming the bridge is not OB, you mark your lie on the bridge and proceed with the hole. If your disc lands under the bridge, you play it from under the bridge, taking any OB into consideration as you normally would. Of course, the TD or course designer is free to make any or all of the bridge OB, in addition to the creek below.