Any county may authorize the issuance of bonds pursuant to title 9, chapter 21, for the construction of county highways, roads and bridges and may irrevocably pledge, during each fiscal year, moneys received by way of state aid grants for rural roads, an amount not exceeding fifty percent (50%) of state aid funds derived from the distribution of the gasoline tax for rural roads, to the retirement of principal of and interest on the bonds, but in computing the amount of state grants, state funds used in matching federal funds shall be excluded.