Very nice set. One advice, that can make the icons a tiny bit better: The shadow effect separates an icon from background and it can also be used to separate the objects inside one icon (this technique is mentioned in the article about shadows in the Internal Shadow section). If you for example take the District.ico and leave the buildings as 3 separate .u3d files, then duplicate the snapshot in your .r3i file and and put each building in its own snapshot with same light and camera settings and the use the "Create Layered Icon" button instead of the normal one, RWIE will create the internal shadows for you.

Thanks for the icons, simple and beautifull...