Section 31. 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:

(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).]

For the purposes of this section, the following words shall have the following meaning:—

(1) ''Streets and roads in such city or town'', the total mileage, to the nearest one hundredth of a mile, of public ways, other than state highway, in such city or town, as determined by the most recent mileage survey conducted by the commissioner of highways; and said commissioner is directed to conduct such survey annually.

(2) ''Motor vehicles per road-mile'', the total number of motor vehicles registered and garaged in such city or town, exclusive of re-issues, as determined annually by the registrar of motor vehicles, divided by the total mileage, to the nearest one hundredth of a mile, of public ways, other than state highway, in such city or town, as determined by the most recent mileage survey conducted by the commissioner of highways.

(3) ''Equalized valuation per mile'', the equalized valuation of the aggregate property in such city or town subject to local taxation, as most recently reported by the commissioner of revenue to the General Court under the provisions of section ten C of chapter fifty-eight, divided by the total mileage, to the nearest one hundredth of a mile, or public ways, other than state highway, in such city or town.

The commissioner of highways shall annually notify the commissioner of revenue of the amounts apportioned to each city or town under this section; provided, however, that the sum payable to each city or town shall be used solely to reimburse such city or town for the costs actually incurred during the fiscal year of such city or town in constructing, maintaining and policing city or town streets or roads and shall not exceed the amount so incurred by such city or town.

The mayor of each city and the selectmen of each town shall notify the commissioner of highways in writing of the amount that will be incurred during the fiscal year of such city or town for constructing, maintaining and policing city or town streets or roads. The commissioner of highways shall annually, on or before December first, certify to the commissioner of revenue the amounts approved for payment to each city or town under this section in each fiscal year.