Officials and employees of state agencies shall, under regulations adopted by the commissioner of administration, and whenever the mode of transportation is authorized or approved as more advantageous to the state, be paid an amount, instead of actual expenses of transportation, not to exceed a mileage allowance set by the commissioner of administration for the use of privately-owned automobiles or airplanes, when engaged in official travel inside or outside their designated posts of duty or places of service. In addition to the mileage allowance, there may be allowed reimbursement for the actual cost of ferry fares, and bridge, road, and tunnel tolls. When two or more officials or employees are traveling in the same direction, and it is possible to share a privately-owned automobile or airplane, the mileage permitted shall be allowed only once.