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MUTCD Section 2A.14 Word Messages
Standard: All word messages shall use standard wording and letters. Word message warning signs are also diamond shaped yellow signs with a black border. Lettering shall be in capital letters as provided in the “Standard Highway Signs” book, except as indicated in the Option below.

Guidance: Word messages should be as brief as possible and the lettering should be large enough to provide the necessary legibility distance. A minimum specific ratio, such as 25 mm (1 in) of letter height per 12 m (40 ft) of legibility distance, should be used. Abbreviations should be kept to a minimum, and should include only those that are commonly recognized and understood, such as AVE (for Avenue), BLVD (for Boulevard), N (for North), or JCT (for Junction).

Support: Some research indicates that a ratio of 25 mm (1 in) of letter height per 10 m (33 ft) of legibility distance could be beneficial.

Option: Word messages on street name signs and destinations on guide signs may be composed of a combination of lower-case letters with initial upper-case letters.

MUTCD -Section 2C.01 Function of Warning Signs

Standard: The use of warning signs shall be based on an engineering study or on engineering judgment.

Guidance: Warning signs should be placed so that they provide adequate PIEV time. Warning signs should not be placed too far in advance of the condition, such that drivers might tend to forget the warning because of other driving distractions, especially in urban areas. Minimum spacing between warning signs with different messages should be based on the estimated PIEV time for driver comprehension of and reaction to the second sign.The effectiveness of the placement of warning signs should be periodically evaluated under both day and night conditions.
The use of warning signs should be kept to a minimum as the unnecessary use of warning signs tends to breed disrespect for all signs. In situations where the condition or activity is seasonal or temporary, the warning sign should be removed or covered when the condition or activity does not exist.

Support: Warning signs call attention to unexpected conditions on or adjacent to a highway or street and to situations that might not be readily apparent to road users. Warning signs alert road users to conditions that might call for a reduction of speed or an action in the interest of safety and efficient traffic operations.

MUTCD Section 2C.15 ROAD NARROWS Sign (W5-1)
Guidance: A ROAD NARROWS (W5-1) sign (see Figure 2C-3) should be used in advance of a transition on two-lane roads where the pavement width is reduced abruptly to a width such that vehicles might not be able to pass without reducing speed.

Option:Additional emphasis may be provided by the use of object markers and delineators (see Chapters 3C and 3D). The Advisory Speed (W13-1) plaque (see Section 2C.46) may be used to indicate the recommended speed.