Bridge and Overpass Segmentation Elevation (BOSE)
| This information is under consideration for the Illinois state wiki. It should not be considered official guidance. |
Please note that there are regional and national discussions ongoing for establishing standards beyond state level. If those changes come in opposition to what we are doing here, then we will be changing to follow suit.
Illinois follows a state standard for the segmentation of bridges and overpasses.
The National Wiki contains the standard information regarding proper elevation of segments. What we are doing in Illinois is expanding on those standards to create a more accurate representation of the segments.
Basic elevation is determined using the following rules:
- All roads start at Ground elevation
- If a segment is above another segment, it should be set to +1 relative to the lower segment's elevation (to account for multiple segments overlapping)
- If a segment is a tunnel (not just an underpass), it should be set at -1, and Tunnel should be checked.
Bridges
Definition:
- If a segment travels over a river or body of water, it is considered a bridge
- If a segment travels over another segment, it is considered it a bridge (aka: overpass).
- If a segment is artificially elevated to pass over ground level objects and natural features, it is considered a bridge.
Junctioning:
Place a junction at the start/end of the bridge where the land falls away
If there is already a junction within 500 feet of the bridge, do not add a new junction.
Ramps
Ramps should always be set to Ground. The only time a ramp segment is set above or below ground is when it travels above or below another segment as a bridge or a tunnel. This is most often the case in large interchanges, like the new I-290/SR-390 interchange in Itasca. When a ramp traverses over or under another segment, you should follow the elevation guidelines listed above.
Do not segment a ramp like you would for a bridge, unless it takes on several different elevations.
Because of the complexity of interchanges, please contact your State Manager before making adjustments.
Special Note
This process was derived from the Seagull Effect created by the Australians. The process has been unofficially called ‘seagulling’. Please note that this word is a misuse of the Seagull Effect. Using it may have unintended connotations.