I propose that primary be used for major state highways (2 lanes each way, heavy traffic, see NY 5/Central Avenue through Albany and NY 7 from Latham to Schenectady for examples), and secondary for lesser state highways. It seems to me that tertiary naturally matches up with county routes and with other 2 lane roads which are wide and fully striped (white on sides of road with breaks for side streets, solid or dashed lines separating the two lanes.) --nfgusedautoparts 16:09 2009-10-30 (EST)
Trunk roads should probably have limited or no connections to local businesses/homes, and i would think are very likely to have frontage roads to allow for such connections. Things in NY that I think are Trunk roads: Taconic Parkway (grade crossings), much of the North-South Arterial in Utica and related roads, and Washington Avenue Extension in Albany (limited grade level crossings w/frontage roads).
I've been using the following rough approach: Primary: US highways and NY highways with 2 lanes each way or more (unless trunk or motorway). Secondary: NY highways, major roads in city (one lane each way). Tertiary: County Routes, or roads comparable to County Routes in cities/villages.