There is a "bug" some of you should be aware of which seems to work like this:
If you're working on your bridge and you save it, then continue to work on that if you then test and finish it says "next level" but it doesn't seem to take you to the next level at all. The way to get to a next level is that every time you hit "test" it saves the bridge in that level...I suck at explaining but the basic gist of what I'm trying to say is that you never need to save a bridge, because each time you hit test it saves it, then when you want to come back to it go start game > level > and that bridge will be there. If you then beat it then you can go to the next level. IE: "TEST SAVES" are the only sort of saves that actually work to get you to next levels; all saved bridges do is let you play on that single particular level.