Properly, that is a state issue. All highways are state highways (yes all of them, including interstates and US highways, those just get federal funding under the “post roads” clause). The state sets the rules of the road. If farmers are driving in an unsafe manner the state needs to rewrite the rules for that state.
The jackasses driving the farm machinery just don't follow the laws. I don't know if they're ignorant of the laws or just too inconsiderate to care. I hate to see more regulation - just the same as anyone else, but years of following idiots driving farm machinery, almost getting killed by idiots driving farm machinery (running stop signs), and occasionally getting run on to the shoulder by idiots driving farm machinery has reduced my sympathy toward them to about zero, and I suspect that A lot of other people won't be very sympathetic to them for the same reasons.