instead of having dirt road/4x4 trail be a road type, make it a checkbox that will make the road display in a distinct color on the screen.
Reason: in my opinion a dirt road could be either a street or a private road. However if you use private road type to prevent routing it shows on the map as a street, although it may also be unpaved.
Way I'm envisioning it is maybe in the schema you'd just have one parameter for dirt road color, and then have that color replace the "color1" that would be normally used for the road type when the dirt road flag is set.
That way if you had an unpaved road of any type, the thickness, declutter, etc. would all remain consistent with the road type, but you could set it to a tan-brown color to indicate to the user that it was in fact an unpaved road. I'm thinking of instances like the Alcan Highway where you've got what might be classified as a major/minor highway while yet being unpaved in portions. (I realize that that's not a valid example as it is now apparently completely paved, but you get the idea.)
I do realize that this idea would require a significant rework, but I'm just throwing it out there because I think it would be a real improvement.
Just a thought...