I think that these shapes are actually just really thick lines, and not
actual road-outlines.
You can change the color of the road by changing the LINE color. The line
color used to be hooked to the fill color, so that users wouldn't have to
know that the shapes are just thick lines, but it seems to be broken now.
If you make the line color none or white, you won't see much. I would
suggest trying to work with a very light gray, or choosing black, but adding
transparency to the line, so that you can see objects in the background. You
can set transparency through the dialog under Format > Line.
