When you render out your model, you can also make different kinds of renders, including a Z-Depth pass. This creates a greyscale image with areas in the foreground dark, and going back in space goes along a gradient to white. Search online for how to create this in Keyshot, V-Ray or even SketchUp by just using ‘fog’. In Photoshop I use this to mask overlays, or create fog with selections, enhancing the overlapping of forms in space. This enables me to quickly select the areas behind the bridge so I can boost the contrast and add depth.