Typically they use 1900 and there are some bridges built in the very late 1800's. That's why the 1801 struck me as odd.
The NBI date is a default date for an unknown construction date. Normally if the date reads 1900 or 1901 and it is of concrete construction that simply means the bridge construction date is unknown. Maybe in Illinois they use 1800 or 1801 since there is a chance of having pre-1900 bridges there, unlike here in Oregon.
The FHWA bridges inspection coding guide states..
“Record and code the year of construction of the structure. Code all 4 digits of the year in which construction of the structure was completed. If the year built is unknown, provide a best estimate.”
I would say the best estimate was not applied here. I would guess from the 1930’s or 40’s
It looks a lot like this bridge >>>