Right now, it appears that the owner of abandoned county road is the county.  As long as the owner of the property, i.e. the county, does not want to sell it, then you cannot buy it.  However, if the county commissions vote to sell the land to you, then they are permitted to do so.  Instead of just asking the head of the road commission, ask to be put on the agenda at the next commissioner's meeting to present your argument for purchase of the land/road.  If it is truly abandoned, then this may be a cost-effective fund raising option for the county commissions.  The commissions can generally trump department heads.