The portion of the Highway Fund allocated for reimbursements to cities and towns for costs actually incurred in constructing, maintaining and policing city or town streets or roads shall annually be apportioned among the several cities and towns as follows:

1. (a) The amount apportioned to each city and town in any calendar year shall be the amount of the ''equalizing municipal highway grant'' for such city or town multiplied by the number of miles of streets and roads in such city or town. The ''equalizing municipal highway grant'' for such city or town shall be the total of a basic mileage allowance of four hundred dollars plus an additional road-use allowance of seven dollars per motor vehicle per road mile less an equalizing deduction of ten cents per thousand dollars of equalized valuation per mile.

   [There is no clause (b).]