In New Hampton (at the junction of US 18 and US 63), Alvin crosses West Hale Street before he approaches it.
Early in the film it was established that it is "harvest time". As Alvin is on his journey a slew of bicyclists pass him and later he motors into their camp. The only explanation for this event would be RAGBRAI, which happens in mid-late July.
The credits state that Sissy Spacek was the first in order of appearance. In fact it was the next door neighbor,, played by Jane Galloway Heitz.
Alvin is shown crossing the Mississippi River on the Black Hawk Bridge at Lansing heading westward into Iowa, instead of eastward into Wisconsin. This bridge, in real-life, isn't in the vicinity of Prairie du Chien; rather, it would have been the Marquette-Joliet Bridge that he crossed.
While driving through West Bend, Iowa, Alvin is seen driving past the Grotto of the Redemption from right to left. This would be north to south, or following the jog in the highway, traveling east to west.