23 C.F.R. § 750.107
(a) The erection or maintenance of the following signs may be permitted within protected areas, outside informational sites:
(b) The erection or maintenance of signs permitted under paragraph (a) of this section may not be permitted in any manner inconsistent with the following:
(1) In protected areas in advance of an intersection of the main-traveled way of an Interstate highway and an exit roadway, such signs visible to Interstate highway traffic approaching such intersection may not be permitted to exceed the following number:
| Distance from intersection | Number of signs |
|---|---|
| 0-2 miles | 0. |
| 2-5 miles | 6. |
| More than 5 miles | Average of one sign per mile. |
The specified distances shall be measured to the nearest point of the intersection of the traveled way of the exit roadway and the main-traveled way of the Interstate highway.