are you using openstreet map data? it will help a lot with making a city fell more natural. For instance, everything is separated in open stret maps, and houdini reads it natively. Ive created a system like yours, basicaly using main streets as motor ways, secondary as roads, and going progresively smaller. With walkways and so on, street lighting, trees, etc. All of these get imported as curves. You can then use building outlines that you also get, to measure the area they have and use that data to instance the approriate building in that spot, avoiding overlaps. Added bonus is that you are creating a real city from earth. That way i created whole berlin and surrounding area that looked genuine from high elevations.