Regulatory Sign Usage
Section 2B.01 Application of Regulatory Signs Standard:
(01) Regulatory signs shall be used to inform road users of selected traffic laws or regulations and indicate the applicability of the legal requirements.
(02) Regulatory signs shall be installed at or near where the regulations apply. The signs shall clearly indicate the requirements imposed by the regulations and shall be designed and installed to provide adequate visibility and legibility in order to obtain compliance.
(03) Regulatory signs shall be retroreflective or illuminated (see Section 2A.07) to show the same shape and similar color by both day and night, unless specifically stated otherwise in the text discussion in this Manual for a particular sign or group of signs.
(04) The requirements for sign illumination shall not be considered to be satisfied by street or highway lighting.
Source: https://mutcd.fhwa.dot.gov/pdfs/millennium/06.14.01/2bndi.pdf