"Black on Yellow" on button copy

[Mergingtraffic]

I've noticed on old button copy BGSes, the black on yellow portions of the signs such as "Exit Only" or "Left Exit" parts are not button copy.  Any reasons why?

[agentsteel53]

sometimes the yellow is retroreflective. 

in the case of old California porcelains, the signs originally did not have buttons.  I do not know why the buttons were not pasted onto the EXIT ONLY when the signs were retrofitted, because other yellow/black signs contemporary to the retrofit (and earlier) certainly had buttons.

I think the main aversion is that the signs are positive contrast, while reflective buttons are negative contrast, and the two put together tend to clash.  that is also the reason CA had so many black signs with white legend in the past; because the addition of discrete reflectors in the white made a lot of sense.

[Alps]

Pennsylvania used yellow reflectors for truck-related messages.  I've seen green reflectors used for the NJ Turnpike and red reflectors used for warnings.  So yellow reflectors for Exit Only seems like a reasonable compromise.