Well, unless you are good at designing trusses or bowstring arches, a simple pole bridge would be your best bet. Utility poles come in 3 standard sizes- 30 foot being shortest. 3 of those 3 feet apart, decked across with 8 feet 2x4s, simple, not an elegant solution but hard to screw up. But have you priced lumber lately- and will the tree huggers allow you to bridge if culvert is a no-no?
What we have here is... failure- to communicate.