Every motor carrier of persons for hire traversing the highways of the state, subject to the provisions of an act known as the Alabama Motor Carrier Act of 1939 [Chapter 3 of Title 37], shall pay to the State of Alabama into the fund of the state Department of Revenue as contribution to the maintenance, repair, and policing of public highways for each mile actually operated within the state on such public highways, whether such vehicle is loaded or empty, a mileage tax of one-fourth cent per mile on all passenger vehicles with a seating capacity of not less than nine nor more than 16 passengers; a mileage tax of one-half cent per mile on all passenger vehicles with a seating capacity of not less than 17 nor more than 21 passengers; a mileage tax of three-fourths cent per mile on all passenger vehicles with a seating capacity of not less than 22 nor more than 25 passengers; and a mileage tax of one cent per mile on all passenger vehicles with a seating capacity exceeding 25 passengers. “Seating capacity,” for the purpose of this chapter, means the actual number of regular passenger-carrying seats in a motor vehicle; and “regular passenger-carrying seat,” as used in this chapter means a seat ordinarily and customarily used by one passenger but does not include folding or collapsible emergency aisle seats; provided, that the number of such folding or collapsible aisle seats shall not be greater in number than 25 percent of the regular seats in such vehicle; and provided further, that, where any seats in the vehicle are not distinguished by separate cushions or backs, one seat shall be counted for each 18 inches of space on any such seat. The taxes imposed by this chapter shall be increased 50 percent if the annual license tax prescribed by the laws of this state on like vehicles is not paid. Such mileage tax shall be paid by such motor carrier in addition to all property, franchise, license or other taxes, fees and charges now or hereafter provided by law. The tax levied shall constitute a debt due the State of Alabama and may be collected by civil action in addition to the methods provided in this chapter.