I forgot to supply the theory & motivation behind it. Here we go.
Most distinguish near trees and distant ones. (I use tree, bush and shrub interchangeably here.) Near trees must look really good as they stand right at the track. The drivers see them from two opposite angles 90 % of the time, so non-rotating 2D trees would be an option if no one ever peeked out the side window.
I am not a fan of rotating trees (upright viewer-facing images of a complete plant) as I find it disturbing. In contrast, I find trees that are composed by many viewer-facing planes quite convincing. The only issue occurs in the moment in which one image goes before the other due to the rotational movement. The tree "comes to life" in an unnatural way, attracting attention to the visual deficit by the unexpected motion/change in the picture. This effect is defeated by shrinking the images so they don't overlap. But then you need a second row behind them and the relative movement of the parts as you pass by can be obvious/disturbing or not. This needs exploration in detail as it largely depends on the textures used.
For distant trees, one could get tempted to model them as solid balls of irregular shape. But the vertex distance would need to be near 1 pixel in the picture the player sees, so you really need a load of polygons. So for distant trees just like for near ones, you cannot avoid 2D stuff with alpha if you want to get the silhouette right. And the silhouette is what stands out. You will not note much of the texture inside the green area that represents the tree in the final picture, but where the tree has a brown or blue background, the observer will note every pixel at the border between tree and background.
One thing to note is that voxel shading is an old hat. It works by creating a 3D image from a stack of 2D images with transparencies. Imagine a stack of static images, spaced by 10 cm, representing a dissection of the tree at that height. If you look don't look straight from the side, but from somewhere above, the superimposition of all the slices will create a convincing 3D image, or so they say. The problem in our case is that you do look from the side. You could use upright images, but there will always be a case where you look at the plant from an side that looks really bad (i.e., where you see the stack of planes from the side).
That said, I see two options for plants of all sorts: Rotating parts and non-rotating.
Non-rotating versions are for another day and also not really new. In the case of a hedge, you would have a row of upright, static planes, spaced by 1 m, that show the dissection of the hedge at that place. The important parts are the outer 1 or 2 m. Stepped in from the outskirts of the hedge by some 2 m or 3, you need a solid shape that covers the cases in which you see the hedge from the side. You also need a plane that runs through the middle of the hedge and carries the image of the top part for those who view the hedge from the side. The solid part is optional, but I find it very important that at the bottom, the solid shape - or another semi-transparent plane - runs right to the margin of the hedge base. You see loads of 2D trees that look decent as long as you don't see their base, showing the very obvious border of the upright image and the totally different ground texture.
The rotating version combines the theoretical elegance of voxel shading with that of viewer-facing geometry. The downside is that both left and right halves of the texture are the outside ones at some point. This means that the texture can only show rather generic leaf-shaped dots without much orientation.
While voxel-based representations need the whole body to be filled with voxels, you can make it hollow when using view-facing quads. If the non-transparent area of the texture is rather small and/or the quads don't overlap, you will see through the near layer of quads. You will see the other quads that form the other side of the hedge for the bigger part of the area. You might want to have something in the middle of the hedge that is gapless and/or a bit darker like the inside parts of the bushes normally are.