Road Network from Curves.

Been working on a urban design definition. The idea is to have road centerlines as a hierarchy of curves in Rhino (or generated in another GH def). Take those centerlines, offset by a different distance for each type of road, trim at the intersections and join the outlines of each plot.

So far so good. 

Next, at each intersection, I want to fillet the corners, with the radius determined by the larger road.

This is giving me trouble. Any idea where to start? 

Note - this doesn't need to be a fillet w/ a true arc. It could be a tangent curve, starting at a different distance from the corner on each adjacent cell boundary line....