Illinois: Round Mile Markers, Interpretations and Misinterpretations

Started by Rothman, April 23, 2024, 04:57:35 PM

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A little late to the discussion (been a bit busy...)

My understanding was that these are intended for IDOT use in defining locations on state highways and state maintained roads. With not all state maintained roads being part of the numbered system they needed a way to identify the locations.

When used on a state or US numbered highway the route number will appear at the top. This typically includes the "BUSN" designation for Business Routes, I have seen that on parts of Business US-24 near Peoria for example. Most of the time the "IL' or "US" designation is not used but there are many exceptions.

On state maintained roads that are not part of the numbered system, often including bypassed or decommissioned roads, an alternate number is used. This tends to vary by IDOT District. Out near Monmouth (D4) for example, parts of US-67's original route are marked as "8967". Other areas, such as the far southern tip of the state (D9) or Peoria area (D4 again) tend to use 900 series numbers. Apparently these are assigned an an ad hoc basis and some numbers are repeated in other districts.

These were never intended to be used as anything other than reference markers, perhaps as part of an implementation of a GIS system. I recall setting up our GIS system, we had to assign names or numbers to things that never had or needed them before. In Kentucky my father in law finally had an address assigned to his home after 50 years as a result of GIS.

I originally thought they were "Inventory Markers" as one IDOT employee told me but apparently they are not included on the IDOT maps containing marking such as "FAP", "SB", "HB", "FAS" etc. inventory series numbers.

The use of these signs are usually in rural areas rather than in town, although with sprawl they can end up in town. There is one or more along IL-83 near North Avenue (IL-64) for example.
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