§ 391. Rates of toll on motor vehicles. Where a different rate is not otherwise prescribed or permitted by law, any person or corporation maintaining a plank road, turnpike road or bridge and authorized, or which shall be hereafter authorized, to receive tolls for the passage of motor vehicles over the same, may charge and receive for each and every motor vehicle propelled by any power other than animal power, passing over the same, a toll rate not greater than the maximum rate allowed by law to be charged and received for the passage of a vehicle drawn over such road or bridge by two animals, provided that for such motor vehicles designed to carry only two persons the rate of toll charged or received shall not exceed the maximum rate allowed by law to be charged and received for the passage of a vehicle drawn over such road or bridge, without a load, by a single animal.