There is a plug in that lets you convert parts to terrain. you can angle a part so it lines up with your road and henceforward convert that part to a terrain type of your choice.
I’m pretty sure the plug in is called part to terrain converter but I could be wrong, either way you will find it if you search for " terrain " in the plug in page.
Try using a plugin called Archimedes Two it pretty much can use parts or models and makes angles by clicking the render button and you can tell it to where to rotate. That’s what I use for my roads rn.