User:Hawkeygoal/IL Pages/IL OSE (2016 07)

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Revision as of 15:31, 15 February 2017 by Hawkeygoal (talk | contribs)


The goal of this page update is add additional guidance on the handling of elevation relative to overpasses in Illinois by revising the Major roads section of the Illinois wiki.

This information is under consideration for the Illinois state wiki.
It should not be considered official guidance.


True Elevation

In Illinois, an expanded implementation of the general guidance for elevation and overpasses, underpasses, and bridges, has been adopted. Please refer to the these sections for additional guidance.

True Elevation refers to layers of road decks supported above and below the terrain (ground) with open space between them. In essence, any road segment which a bird, such as a hawk, could fly underneath should be elevated above ground level. The inverse condition applies to tunnels or multi-level underground segments where your average groundhog could tunnel between them.

What is an overpass? An overpass is a general term for any road, walkway, railroad, bridge, or other mapped structure crossing over an impediment such as a waterway, natural formation, or another road, walkway, railroad, or bridge. In a mapping context it can also be conceptualized underground where the crossing is supported by surrounding earth.

These elevation rules apply:

  • All surface roads when not supported above or tunneled below the terrain are set to ground elevation unless otherwise described in this section.
  • If a road segment is supported above the terrain or another road segment, the elevation of the segment should be set one higher (+1) than the segment or terrain (ground) immediately underneath.
  • The elevation of road segments completely or transitionally sub-surface should be negative and one less than (-1) the surface, tunnel, or level immediately above.
  • Ramps should be set to the lower elevation of the roadways to which it junctions unless they themselves pass over or under other segments as described above.
  • Please contact a State Manager before making elevation changes on freeways or interchanges (including connecting ramps and segments).

When creating segments to implement this guidance, please observe these precepts:

  • Create junction nodes for elevation purposes at the point the physical change in elevation occurs unless otherwise guided in this section.
  • Use existing junction nodes when within 500 feet of an elevation change. For example, do not create new junction nodes for elevation purposes at an interchange when ramps junction with the main road within 500 feet of the physical overpass.
  • Do not create an elevated overpass less than 150 feet in length (with a strong preference for 200 feet).
  • Contact an SM or LAM for approval to create overpasses, or single junction nodes for elements less than 150 feet in length (overpasses, culverts, etc.).

A culvert is a tunnel allowing a stream or open drain to pass under a roadway. Under normal circumstances these are not to be addressed by True Elevation mapping. However, an elevated segment for the culvert may be created to support real-time closures (due to flooding or repair) when all these conditions are met:

  • Is of significant construction requiring multiple day closures to repair.
  • Is not located near the center of the segment.
  • The segment is heavily populated by potential destinations and a route to the wrong end of the segment would result in a manual detour of more than one mile.


This proposal is still being drafted prior to open discussion.