George,
If you go with wood, remember that the main member strength needs to quadruple as the span doubles.
For example, 4 treated 2x12's would be strong enough bracing for a 10' bridge, and carry an ATV and 2 riders just fine. At 20 feet, (if you could even find 20' 2x12's) would be way too bouncy, and may even break.
That means that a 20' span would need to be massively strong, or (preferably) have 1 or 2 pair of posts.
