I don't know the GIMP, but in Illustrator this is best done by having a black line, for the road casing, at say 1.2 pts, and then copying it and pasting above on a new layer, coloring it red, and making it 0.6 pts. This would give you a red road with a black casing.
For a red-white dash with a black casing, simply copy your red line and paste it above on a new layer, and set it to white and dash it.
-Rob