23rd
National Standards for Directional and Other Official Signs Within the Interstate System
The following signs are prohibited. Signs advertising activities that are illegal under Federal or State laws… Signs which are erected or maintained upon trees or painted or drawn upon rocks or other natural features. Obsolete signs. Signs which move or have any animated or moving parts.
The message on directional signs shall be limited to the identification of the attraction or activity and directional information useful to the traveler in locating the attraction Descriptive words or phrases and pictorial or photographic representations of the activity or its environs are prohibited.
Attractions eligible for directional signing are limited to the following: natural phenomena; scenic attractions; historic, educational, cultural, scientific, and religious sites; and outdoor recreational areas. To be eligible,… attractions or activities must be nationally or regionally known, and of outstanding interest to the traveling public.
Hats off to Lee Sandlin
