I'd like to suggest the following changes be added into the Major Roads page.
True Elevation
In Michigan, an expanded implementation of the general guidance for elevation and overpasses, underpasses, and bridges, has been adopted and follows Illinois rules. 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.
Elevation
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.
Segmentation
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 a 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.
Culverts
A culvert is a tunnel allowing a stream or open drain to pass under a roadway. Under normal circumstances these are not to be mapped. However, an elevated segment for a culvert may be created to support real-time closures (due to significant flooding or repair) when all these conditions are met:
- The culvert is of significant construction requiring multiple day closures to repair or has major flooding potential.
- The culvert 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 un-routed detour of more than one mile.