(2) SPEED ON COUNTY ROADS.--The maximum speed on any county-maintained road is:
(a) In any business or residence district, 30 miles per hour in the daytime or nighttime; provided that with respect to residence districts a county may set a maximum speed limit of 25 miles per hour after an investigation determines that such a limit is reasonable; and it shall not be necessary to conduct a separate investigation in each residence district.
(b) On any other part of a county road not a business or residence district, as set forth in s.
316.183.
However, the board of county commissioners may set speed zones altering such speeds, both as to maximum and minimum, after investigation determines such a change is reasonable and in conformity to criteria promulgated by the Department of Transportation, except that no such speed zone shall permit a speed of more than 60 miles per hour.