A description of how things work at this time.
The terrain dustiness is a result of combining the tile's dustiness value (from the map editor) and the ground saturation.
saturationDustFactor = (1 - saturation / 4%) or 1, which ever is lower.
finalDustiness = saturationDustFactor * tileMapDustiness.
at 0% saturation, finalDustiness = 100% of the tile's dustiness.
at 1% saturation, finalDustiness = 75% of the tile's dustiness.
at 2% saturation, finalDustiness = 50% of the tile's dustiness.
at 3% saturation, finalDustiness = 25% of the tile's dustiness.
and at 4% saturation, finalDustiness is 0%.
The finalDustiness must be greater than or equal to 25% in order for it to be kicked up be vehicles.
Once the finalDustiness is found, it is then modulated against the vehicle speed to determine the scale and transparency of the dust.
I'll discuss this with the other developers to see if we can create a better solution.