Building a small bridge?

I plan on building a small bridge about 20 feet long over a creek that can support a car. It will hardly ever have a car on it but I want to be able to drive across it when I have to. Without getting into discussions about the existing soil conditions and permits, What I need to know is this: if I build 2 beams about 6 feet apart (where the wheels of the car would be) out of several treated 2x12s or 2x14s, how many 2x12s or 2x14s do I need to bolt together so they won't break. Don't worry, I will add lumber between the beams to finish the deck. Thanks.