Almost all roads you can find on the game map. And don't miss that little guy:

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Ok, if there no grey roads left, take a look on stadium (north)
and Brickchester

When you ride on it, you can see more roads on your minimap.

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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!