First, can you verify the product and version you are using please?
I am assuming you are referring to a clear distance between girders longitudinally down the bridge? The template you create can have closed components that do not necessarily touch each other. So if you create these closed components in the template side by side with the gap between them, you can display the components of the surface to get the 3D shape.
It may be beneficial to also attach some screen shots of what you are seeing in the Roadway Designer and provide some detail or an image of what you would like to model.
You may also want to check out this article on creating bridge typicals:
