I assume that you hope for something like this ?
Here’s how I did it…
First I Purged unused items from the model using Model Info > Statistics.
I gets much smaller.
Then I also corrected issues with faces/edges on Layers other that Layer0 - the container group should use Layers to control its visibility, but its geometry should be on Layer0.
Next I selected the flat face and right-click > context-menu > Texture and ensured that it was ‘Projected’.
Next I edited the surface group and selected all, then right-click > context-menu > Smooth…
In that dialog I set it to apply to all edge types and at maximum slider value - that way it’s definitely a single unbroken surface.
Now, still in the edit, I opened the Materials Browser > Model and used the Eyedropper tool to sample the projected texture from the face outside of the group.
I then used it to paint the surface in one go.
As you can see, the surface then has a single projected texture - although of course the road bridge oddly follows the surface as it has no other option !