The maximum vehicle speed limit set forth by the California Vehicle Code on County
highways or portions thereof within the unincorporated territory of the County of
San Mateo shall be decreased by Ordinance of the Board of Supervisors to prima facie
vehicle speed limits of 25, 30, 35, 40, 45, 50, 55 or 60 miles per hour, whichever
is found most appropriate, to facilitate the orderly movement of traffic and is reasonable
and safe. Said prima facie speed limits shall be effective when appropriate signs
giving notice thereof are erected upon said County highways or portions thereof.