Here is a simple tutorial on how to find all lego roads in Forza Horizon 4.
Ok, if there no grey roads left, take a look on stadium (north)
When you ride on it, you can see more roads on your minimap.
And finally! the most tricky ones. Go to Brickchester and look closely to your minimap. There are slightly grey, very thin lines over a white roads. Well, that’s all!