Illinois/Highway Classification

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This page will provide guidance to IL mappers in consistently classifying highways according to the US Highway Classification standards.

Other Illinois-based editors are encouraged to engage on the discussion page here, on the OSMUS Slack, or anywhere else, in order to move toward a general consensus.

Highway Classes

Fro the Chicago metro area, see Chicago_Area/Highway_Classification


From the US Highway Classification page:

The top category, highway=motorway should only be applied to roadways that are either signed Interstate highways or have ALL of the following characteristics:

  • Grade separation
  • Limited access via on/off ramps
  • No at-grade intersections or traffic signals (although on-ramps may have traffic control devices)
  • Divided carriageways
  • Designed and maintained to support high speeds over long distances as part of an interconnected motorway network

In Illinois, there exist many roadways which exhibit motorway characteristics, but only for shorter distances. See, for example, the US 20 bypass around Freeport.

42°17′58.56″ N, 89°38′08.16″ W

Following the US Classification guidelines:

Roads which are disconnected from the motorway network, but briefly exhibit motorway-like characteristics for short distances (also known as "motorway islands"), should not be tagged as a motorway. In general, a disconnected motorway should have multiple grade-separated, controlled access interchanges over a significant distance, generally at least 2-10 miles, in order to be tagged as a motorway island.


Roads that exhibit most of the characteristics of a motorway. Roads that would otherwise be motorways but have at-grade intersections or extend only short distances may be classified as trunk roads.


  • US 51 between Bloomington and Decatur
  • Cedar Street Bridge between Peoria and East Peoria



The main non-trunk/non-motorway interconnecting roads between large communities. These are often US highways but state and county highways occasionally are also classified as primary roads. Rural primary roads are often designed for high speed traffic over long distances but lack divided carriageways.


  • Most parts of US 150
  • Old/Historic Route 66
  • Towanda-Barnes Road in Bloomington




  • Most state highways
  • Important county highways


The most common major intra-urban arterial roads, secondary roads in urban areas tend to extend long distances through multiple neighborhoods and have higher traffic volumes than lower ranked roads. Certain important arterial roads may be instead classified as primary roads.



  • Most county highways


Minor arterial roads. Tertiary roads in urban areas tend to be smaller than secondary roads in volume and/or distance. Functionally, tertiary roads often link neighborhoods/subdivisions to nearby secondary and primary roads or serve as minor interconnecting links between secondary/primary roads.

Important Populations Centers for Trunk/Motorway Network

In no particular order, the following populated places are considered large/important enough to include in Illinois' motorway/trunk network.

  • Chicago Metro Area
  • East Dubuque (for connectivity west to Dubuque, IA)
  • Rockford
  • South Beloit (for connectivity north to Beloit / Janseville, WI)
  • Moline / Rock Island
  • Peoria
  • Bloomington / Normal
  • Springfield
  • Champaign
  • Decatur
  • East St. Louis
  • Terra Haute, IN (not in IL, but the biggest population center between Springfield and Indianapolis, which is an important transportation corridor)
  • Mount Vernon
  • Marion (also Paducah, KY and Mount City to the south, for connectivity w/ KY and MO)
  • Effingham
  • Quincy
  • Galseburg
  • Macomb
  • Hannibal, MO (again, not in IL, but an important node just over the border, and needs to be considered)
  • Kankakee

Chicago Metro Area

For simplicity, we'll consider this to be Cook County and the "collar counties" surrounding it, including DuPage, Will, Lake, Kane, and maybe just a bit of Kendall.

Most of this area is quite built up, but around the edges of the metro area, there are distinct centers of importance as well.

  • Chicago itself
  • Aurora
  • Naperville
  • Elgin
  • Waukegan
  • Joliet
  • O'Hare Airport (Not a populated place, but my goodness, if every highway in IL doesn't converge here)


IDOT (Illinois Department of Transportation) maintains the authoritative datasets on motorways and numbered state routes in Illinois, as well as a host of other datasets about local roadways. While these do not directly translate to the OSM highway classification system, they can be useful in identifying important populations centers, especially those not already served by the interstate system.

IDOT FHWA Classification

IDOT classifies roads in the state according to the FHWA system. Current functional classes can be viewed at IDOT's IROADS web app.

Roads Feature Service

Annual Average Daily Traffic

AADT Feature Service

Traffic Counts viewer