If it is decided that any money authorized to be expended from the State Treasury shall be expended as required by Section 7 of Article XVI of the California Constitution, the Director of Transportation shall have the exclusive management and control of the expenditure, if the boards of supervisors of the several counties concerned assent to the relinquishment and transfer of such management and control. However, the Director of Transportation may delegate his powers to the board of directors of the district. The board of directors of the district may vest, in the Director of Transportation, authority to supervise all or part of the work of improvement.