The Commissioner of Highways is hereby authorized and empowered to make acquisition of all of the property rights and interest specified in section five of this article by purchase at private sale or, in the event he is unable to do so, by proceeding in eminent domain. Upon any such taking or acquisition pursuant to the provisions of this article, just and full compensation for the sign and leasehold interest shall be paid directly to the owner thereof, and just and full compensation for the loss of the right to erect and maintain signs shall be paid directly to the owner of the affected real property. In any condemnation proceeding involving such taking or acquisition by the state, the commissioners or jury shall ascertain the compensation to which the owner of the sign and leasehold interest is entitled, separate and apart from the compensation to which the owner of the real property is entitled, as provided and authorized in section eighteen, article two, chapter fifty-four of this code.