came across this fascinating link...
http://michigan.gov/mdot/0,1607,7-151-9620_11154_11188—,00.html
has pics & descriptions of the bridges by county
http://www.historicbridges.org/map_mich.htm
this site is similar but with a lot more details
theres also a couple books on the topic i think
http://www.amazon.com/Historic-Highway-Bridges-Michigan-Great/dp/0814324487
