The part of the Northern Tier we did in Iowa (we detoured to Devenport instead of going to Moline) was, for the most part, moderately to more severely rolling, hotter than hell and even more humid. The portion in Indiana was mostly flat to gently rolling in places and hotter and more humid than Iowa. The low the night we stayed at Fletcher Lake was 85 with no breeze. The next day it hit 107 in Huntington. Illinois was brutal as well.
The drivers in Iowa were very courteous and patient, especially when we were slugging up some steep roller. Maybe that's an effect of RAGBRAI. Or maybe they realize that if they going flying past in the other lane and crest the top of a hill wihout being able to see what's on the other side they might have a head on with a giant combine. In Indiana, on the other hand, a local cashier warned us to be careful at the blind interesctions through cornfields. She told us people often blow through stop signs on such country roads figuring no one will be there.