In my opinion, the county most likely classifies them as temporary materials, and not structures. A structure is typically defined as a man made enclosed space, such as a house, garage, and sometimes a shed, etc.
The county has every right to use the entire 100' right of way as it sees fit to fulfill it's obligations to the public.
As for blocking access, are they blocking your driveway? Typically a county road will only allow so many points of access from a single parcel (usually one point of access ie a driveway), for traffic control and safety purposes. Do you have several driveways coming off of the county road, such as farm roads for example? The county probably views only one driveway on your parcel as a true access, and any others that may exists as a convenient spot to store materials.
I'm sure this is not what you wanted to hear, but you did ask for opinions.

