The DOT’s document archive is a wealth of information, most of the dry engineering type, but also more about signs and construction maps.
Through that, I discovered one thing I never knew and one I wondered about in Iowa’s 400-series highways:
- There was a preliminary plan to extend IA 401 north and then east, across the then-being-built Saylorville Dam and on Oralabor Road. Aside from one very-large file, there’s nothing else about it.
- In the signage plans for I-380’s northernmost segment in 1983, the BGSs included IA 412 with San Marnan Drive. They were crossed out at some point before final erection, though, and 412 was decommissioned by the end of 1983. The initial sign plan also did not put US 20 on the interstate. Instead, the freeway going west from the 380/20 interchange was labeled only as “To 21”. That changed in 1986.
I have included a clip of the sign diagram for San Marnan at 380, including where “End 412” would have gone, on that page.