In planning such system of safety roadside rests, the department is directed to generally follow these criteria:
(a)
Safety roadside rests should be provided so that, in combination with other stopping facilities, there shall be facilities available at intervals of approximately one-half hours normal driving time.
(b)
On highway entrances to large metropolitan areas, safety roadside rests may be provided.
(c)
On high-volume highways consisting of four or more lanes, safety roadside rests should be provided on both sides of the highways; on all other highways only single safety roadside rests should be provided.
(d)
Notwithstanding the provisions of subdivision (a), on high-volume highways more safety roadside rests may be planned at strategic locations where they appear to be necessary or desirable.