This USA Today article discusses how different states approach historic preservation of bridges:
The article lacks a lot of detail on what "adopting" a bridge entails - in Indiana, it actually appears more like a sale. According to their website:
"Available - These bridges are scheduled for replacement and the Section 106 process is complete. These bridges have a Memorandum of Agreement (MOA) that allows for their relocation and reuse by interested parties. Interested parties should coordinate with the contact names on the individual pages to determine if the bridge is already in storage and ready to transfer or if its dismantling is yet to occur."