Do County Route Markers (M1-6) need to be high intensity?
There are no requirements for minimum retroreflectivity levels for these yellow on blue signs, but they still need to be retroreflective (see the information from MUTCD section 2A.08 below).
Recent research on brown and blue signs appears to support using the same retroreflectivity requirements for blue backgrounds as shown for green. In other words, use the green standards. For white on green guide signs, the green background can be engineering grade, as long as it meets the minimum reflectivity values. For more information, see MUTCD Section 2A.08 - Maintaining Minimum Retroreflectivity.
For common guide signs, such as street names, the white lettering has to be at least ATSM Type II (Super Engineering grade or equal). The minimum values for white (W ≥ 120 cd/lx/m2) are large enough that ATSM Type III (Hi-Intensity) is really a more economical option. The Super Engineering grade will not last more than a few years in the field.
For County Route Markers, the bigger problem is what to do about the lettering as there are no standards for yellow as the foreground color. Because the lettering is a smaller portion of the sign, it typically needs to have a high contrast compared to the background color. While not required, it would probably be smart to use Hi-Intensity for the yellow portion of the signs, especially if the background is engineering grade. In addition, the shape of the sign will also help even as the blue begins to fade, if the lettering is still nice and clear and yellow.
MUTCD Section 2A.08: Retroreflectivity and Illumination
There are many materials currently available for retroreflection and various methods currently available for the illumination of signs. New materials and methods continue to emerge. New materials and methods can be used as long as the signs meet the standard requirements for color, both by day and by night.
Regulatory, warning, and guide signs shall be retroreflective or illuminated to show the same shape and similar color by both day and night, unless specifically stated otherwise in the text discussion in this Manual of a particular sign or group of signs. The requirements for sign illumination shall not be considered to be satisfied by street or highway lighting.