When the revenue bonds issued for the Bridge, or any addition or improvement thereto, and the interest thereon shall have been paid, or a sufficient amount shall have been provided for their payment and shall continue to be held for that purpose, the collection of tolls by the Department for the use of the Bridge shall be reduced to such nominal amount as shall be sufficient, in the judgment of the Department, to provide only the funds required for maintaining, repairing and operating such Bridge, or such collection shall cease entirely if such cessation is then in any way found practicable.