When the commissioner shall determine that a highway reconstruction, relocation, or maintenance project financed in whole or in part by state funds and conducted under department of transportation supervision or control necessitates the relocation of any municipally-owned subterranean utilities facilities, any trenching and backfilling required for such relocation shall be part of the cost of such reconstruction, relocation, or maintenance and shall be provided by the governmental agency which is carrying out such highway work at no cost to the municipally-owned utility, and the governmental agency shall pay the municipality for the book value (original cost less allowable depreciation) at the time the municipally-owned subterranean facility is retired. Provided, however, that if a public utility other than a municipally-owned utility makes use of a relocation trench provided for in this section, said utility shall pay the governmental agency carrying out such work its proportionate share of the cost of such trenching and backfilling. For purposes of this section, the term “municipally-owned utility” means any utility that is owned in its entirety, directly or indirectly, by a municipality or by a department, instrumentality, company, or corporation that is owned in its entirety, directly or indirectly, by a municipality. For purposes of this section, the term “municipally-owned subterranean facility” means any subterranean facility that is owned, directly or indirectly, by a municipality or by a department, instrumentality, company, or corporation that is owned, directly or indirectly, by a municipality.