The boards of supervisors of counties are hereby vested with the same powers and authority as the councils of cities and towns by virtue of the Constitution of the Commonwealth of Virginia or the acts of the General Assembly passed in pursuance thereof. However, with the exception of ordinances expressly authorized under Chapter 13 of Title 46.2, no ordinance shall be enacted under authority of this section regulating the equipment, operation, lighting or speed of motor-propelled vehicles operated on the public highways of a county unless it is uniform with the general laws of the Commonwealth regulating such equipment, operation, lighting or speed and with the regulations of the Commonwealth Transportation Board adopted pursuant to such laws. Nothing in this section shall be construed to give the boards of supervisors any power to control or exercise supervision over signs, signals, marking or traffic lights on any roads constructed and maintained by the Commonwealth Transportation Board. No powers or authority conferred upon the boards of supervisors of counties solely by this section shall be exercised within the corporate limits of any incorporated town except by agreement with the town council. In the County of Fairfax an ordinance may be adopted by the board of supervisors under this section after notice of intention to propose the same for passage has been published in accordance with § 15.2-1427. After the enactment of such ordinance by the board of supervisors, no publication of the ordinance shall be required and such ordinance shall become effective upon adoption or upon a date fixed by the board of supervisors.