I'd be hard pressed to drive ANY miles on I-80 if I lived in Iowa and wanted to get to SD and the Black Hills. (and I live in Iowa) 


From Eastern Iowa (also where I'm at) get to Hwy 20 however you like and head west all the way across Iowa if you want a nice quiet 4 lane all the way to Sioux City and head north on 29 to Sioux Falls.  Hammer down to the west from there. 

Or you can stick to the "Avenue of the Saints" route and take Hwy 27 from Iowa City/Cedar Rapids/Waterloo wherever you pick it up and stay on it till you're on I-35 at Clear Lake and in a half hour you get on 90 west at Albert Lea MN. Also all 4 lane like the above option