The old bridge is 2 telephone poles that are badly deteriorated. So my thought is I need 28' length with ends sitting on 8" of concrete block. Can I use 6 x6 x 16' pressure treated with a 4' lap joint glued and bolted together for a total of 28'? Only 2 x 6 decking will go on top.
What is the question besides trying to build a bridge with pieces of wood and glue.
I would not go over it since I do not know much about it, but spanning 28' is a stretch and the weather will kill it. The decking may be OK if it is only for an occasional bicycle traveling fast to get off before anything happens. You will definitely have a "belly" in the middle.
Sorry to be so short, but that is not much information to go on and give an approval.