What about if you have 1) interstates, 2) highways, 3) roads, and you want to weigh them differently in the density calculation? Also, for a nationwide computation (multiple areal units), I was considering feature to raster, then using zonal statistics to compute the density measure. The problem is that the vectors of all three road types would be the same size/value unless I select something arbitrarily during the feature to raster conversion. I guess that would be when I assign weighted values. Any better ideas? Appreciate any help and guidance.