The tax rate limitations of section 20 shall not apply where an extra tax levy has been approved by the electorate of the county of not to exceed 1/2 of 1% of the assessed valuation as equalized for the preceding year, if such levy has been submitted to the electorate pursuant to a resolution of the board of supervisors approved by the governing bodies of each city within the county. Revenues derived from the extra tax levy authorized by this section may be used for the improvement of city streets in such amounts and on such projects as shall be mutually agreed upon by the county road commission and the governing bodies of the cities and approved by the board of supervisors. The provision of this section shall apply only when the tax rate limitations of section 20 are exceeded.