North side of bridge
Photo taken June 2010 by Jay Rush.
In Minnesota and Iowa respectively, it's all hit and miss. Some counties resort to wooden stringers as a way for the structure to conform to the surroundings and for reasons that the road is sparsely travelled. That is especially common in the northern half of Minnesota where wood is abundant and the area is heavily wooded and populated with lakes. Other counties prefer concrete as a cost-cutting measure without regard to the conformity with the surroundings, mainly because they are looking for a bridge that is functional and not based on aesthetics. This is especially noticeable in counties that are cash-strapped and are looking at ways to cut costs.
In Michigan we have a number of counties that still build timber stringer/slab bridges even today.