You could do that in 2 smaller masks too. Paint a vertical roadmap, repeat it in TG. Add another version, repeated, then rotated, and add a separate bend mask on top of those.
Takes some fiddling. But one complete mask can be done too, in greyscale (LZW tiff), when all is black and only road is white, even a big mask won't take up much MB's. I often make masks of 5000x5000px or so, even in RGB for three masks.
I am now working on a big piece of land with 2m wide ditches, kilometers long. My mask is 6000x6000px with white, slightly blurred (1,5) lines of 2px wide (line function in PS). In TG they are smoothly interpolated anyway, so with a color adjust or bias function you can adjust widths pretty good, and the ditches are straight, and neat as can be.
Or just make the corner, and add simple shapes to that. Lots of ways.