Q:

Blending features in QGIS: Approximating road edges for map render

I'm having trouble getting the right render for the tertiary roads and service roads look in my map render. I'm trying to get road edges like in the old USGS topo maps:

Notice how the road interior is empty. You can see boundaries and contours through the road.  So the trick of using a thick white line with two thin dark edges wont work for me.
So I set up two lines with offsets in opposite sides of the road. My version shows overlaps between road segments:

The rendering in QGIS is incredible, so I'm wondering if there's a style approach that I haven't discovered to get the appearance of connected road edges.

A:

You need to use Symbol levels to have nicely rendered crossing lines. Using symbol levels you can sort the symbol levels and thereby control the order in which the different symbol layers are rendered. Then under the rendering tab, use the multiply Layer to get the effect of seeing the rivers under the road network.
before applying symbology levels:

Using Symbology levels:

Apply Multiply Layer rendering:

It gives the following output:

