This is the second version of my sunken assets collection, now with curves, more options, and water compatibility! It's a big collection, but feel free to pick and choose what you are most interested in. The assets have been assigned custom UI priorities and should show up in the back of your Parks menu.

This collection allows for sunken walls that can be used to contain highways, railways, or anything else! The walls will hide the cliff texture and allow for a very sharp-looking product. They also double as parks, and show up in this category.

I know this isn't a perfect solution, so let me know of any problems you are having and I will try to help you out! It is definitely very finicky but it is better than nothing in my opinion.

Tutorials:

Straight Roads and Intersections: http://imgur.com/a/6ajsT
Curved Roads: http://imgur.com/a/hP3Nw
Corners/Water: http://imgur.com/a/aS93v
Hiding the upper road: http://imgur.com/a/CHNH3
On/Off ramps: http://imgur.com/a/i6DSr

Triangle counts are under 50 for straight pieces and are no more than 750 for the curved assets.
Each asset has a custom LOD that is under 100 tris.
Texture sizes are also very small (512x256) and utilize repeating textures.

Stats:
25 per unit cost
3 per unit maintenance
10 per unit entertainment accumulation
250 entertainment radius
10 tourists per unit

These mods are helpful and/or necessary for some features:

Terraform Tool: Required to change the terrain height and allow below-grade road construction.
Road Anarchy: Required to use sunken intersections.
No Pillars: Required to use sunken intersections.
Enhanced Road Heights: Helpful when building roads with more precise slopes.
Precision Engineering: Helpful when laying out roads for alignment.
Toggleable Whiteness: Helpful when aligning ploppable assets.
Remove seagulls: Parks create a lot of seagulls, which can be annoying. This mod gets rid of them.