23 CFR § 630.404 - Standards.
(a) Highway purposes may best be served by the establishment of station markings for horizontal control along Federal-aid highway routes at spacings of three to eight kilometers (about 2 to 5 miles) and station markers for vertical control of spacings no closer than one kilometer. These requirements may be waived only with the approval of the Administrator.
(b) Projects should be of sufficient scope to permit efficient use of field parties. Projects should extend at least 30 kilometers. Projects may be coordinated with adjoining States to attain greater efficiency.
(c) Where geodetic station markers cannot be established inititally at points readily accessible from the Federal-aid route, or where unavoidable circumstances result in their being established within construction limits, supplemental projects may later be approved to set and survey markers at satisfactory permanent points, preferably within the right-of-way but at points where their use does not introduce traffic hazards.