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

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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 new page is currently undergoing modifications. The information presented should be considered a draft, not yet ready for use. Content is being prepared by one or more users. Do not make any changes before you send a private message to this user.
This a proposal development page for the Illinois state wiki. This page should never be considered official guidance. Please refer to Illinois for official state guidance.

This guidance, while substantially similar to other guidance for other states, attempts to address items of concern within the state. The Illinois proposed Illinois guidance specifically addresses the multi-level subterranean roads present in the City of Chicago, aligns minimum segment length with that of speed limits (for same reasons), and attempts to address culverts that may contribute to substantial detours. In this, we hope to provide clarity in guidance for unique circumstances within the state and minimize the introduction of additional junctions to the map in addressing elevation concerns.


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 our mapping context it can also be conceptualized underground where tunnels crossing over, or under, one another.


Apply these points to the use of true elevation:

  • All roads when not supported above or tunneled below the terrain are set to ground elevation unless otherwise noted.
  • 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 beneath.
  • 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 are to be set to the lower elevation of the roadways to which they junction unless passing over or under other 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 specified.
  • Do not cut a segment within 200 feet (60m) of an existing junction.
  • Do not create an elevated segment less than 200 feet (60m) in length.
  • Underground segments should be cut at least 50 feet (15m) before, or after, the entrance to the subterranean section.
  • Check the tunnel option for any underground segment.

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 a 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 manually-routed detour of more than one mile.


This proposal is still being drafted prior to open discussion.