Will my wooden bridge withstand the weight of my small truck?
From conventional Hemlock Span tables it seems your 4x6s are only good for about 10' span at 40LBS live load.  Your load is much higher when the trucks out in the middle of the bridge.

I think this is what you've got:


Can you get 2x12s? or better, get an engineer to run the numbers.  You will need to calculate the actual load and deflection and not just rely on flooring span tables since your load is very specific and not distributed over the entire span.