I've tried using the linked method to determine area that drains to stream vs drains to road. For some reason when I run the watershed tool on the the raster representing stream and road features, I get a lot of areas that have no data (drain to neither road nor stream). I'm not sure why this is happening.
Picture: stream lines are in light blue, drains to stream in dark blue, road in white, drains to road in green.
at @Jon Filling the origional DEM then re running the watershed tool got me some more coverage as shown in this picture. New area is shown in yellow and rose.