Subject to legislative appropriation, allocation and direction, all net revenues remaining after payment of all refunds allowed by law and expenses of collection derived from motor vehicle registration fees, and all taxes, except gross production and ad valorem taxes, on motor fuels and lubricants used to propel motor vehicles over public roadways, shall be used for the sole purpose of acquiring rights-of-way, constructing, maintaining, and policing such public roadways, and for the administration of such laws as may be prescribed by the Legislature pertaining to the supervision of traffic and safety on such roads; and for the payment of the principal and interest on county and road district bonds or warrants voted or issued prior to January 2, 1939, and declared eligible prior to January 2, 1945, for payment out of the County and Road District Highway Fund under existing law; provided, however, that one-fourth (1/4) of such net revenue from the motor fuel tax shall be allocated to the Available School Fund; and, provided, however, that the net revenue derived by counties from motor vehicle registration fees shall never be less than the maximum amounts allowed to be retained by each County and the percentage allowed to be retained by each County under the laws in effect on January 1, 1945. Nothing contained herein shall be construed as authorizing the pledging of the State’s credit for any purpose.