The roads are linked with trophies from areas which the Iowans have converted and/or conquered. On pilgrimages people bring with them signs (literal in this case) to be place along their path (in Iowa) on their way to [as yet un-picked city]. These are tributes. Trophies of CONQUEST are placed in the opposite direction from which they were taken (a sign taken from the east, would be placed on a western road).