No 'Best', Only Alternatives
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Iowa is currently reporting at two separate closures of portions of I-29 due to flooding in southwestern Iowa. Missouri is reporting a similar closure in northwestern part of that state. I could easily see the detours associated with these closures costing you an hour or more. One alternative would be to take I-80 east across central Iowa to just past Des Moines and then IA-163 to Oskaloosa and US-63 south. At Ottumwa, you'd pick up US-34 east to US-218/US-61 south into St. Louis. Such a detour would also cost you about an hour or so (as well as about 50 extra miles), but I've driven portions of it and it would be a relaxing alternative to dealing with detours and flooding and, particularly in the east along the Mississippi, it can be quite scenic.
AZBuck
US 34 bridge to Plattsmouth?
The Iowa DOT still shows IA-2 from I-29 to the Missouri River Bridge, a segment of only a couple of miles, if that, to be closed. They update in the early morning daily so it appears to be valid intel. Furthermore, they show a detour for IA-2 to reach Nebraska City involving running further up I-29 to US 34, crossing the Mo River at Plattsmouth, NE. From there, it appears NE 66 runs due west on the south side of the Platte River to the northeast side of Lincoln, where one could pick up I-80 and still enjoy something of a shortcut around Omaha.
Foy