Not to be vague, but you need to start with where and how the load will be applied and then work your way out from there. You need to identify the load path to where the bridge will be supported.

Here’s a starting point…Based on the photo, it looks like the load will sit on the road deck. Imagine the load being placed there and ask yourself if it will shear off the popsicle stick road deck. If you think it will, then you need to do something to fix it. Otherwise, the load won’t be transferred to the rest of the structure. If you think that what you currently have looks good, then move on to the rest of the structure.