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A starting point would be to add a empty Layer at the right point in your Layer Stack.  Not that the white roads are above the terrain yet below the Lettering and at least some of the blue lines.

Use the pen tool to to create a Path for the center line for the road

Stroke that Path with the desired color road then apply a layer style with the "Stroke" option of the desired color (black for black outline)

This will get you close yet I noticed that the road also adopts some of the shading of the terrain below it so that was be additional work yet not sure how far you want to go.

I hope this gives you some ideas to  consider.

John Wheeler