It is impossible to unlock new regions with bridges. I can't reveal a new map chunk while standing on a bridge, and I also can't build a bridge in the dark area.
here it is: https://drive.google.com/file/d/0Bw9XQkE78ae6ZHU4VHg0djZlZ1E/view?usp=sharing
i find a workaround by change plotvis in the save file (not in the upper one but in a copy)