How are "blank-out" signs illuminated?

"Blank-out" signs are illuminated internally. This means that the light source that allows the sign to display information or to blank out is contained within the sign itself. Internally illuminated signs use light-emitting diodes (LEDs) or other lighting technology that is integrated into the sign structure.

The advantage of internal illumination is that it allows for smoother and more uniform lighting across the surface of the sign, enhancing visibility to drivers and pedestrians. Unlike externally illuminated signs, which rely on outside light sources to shine on the sign, internal lighting ensures that the sign can be clearly seen even in low light conditions or at night.

In contrast, the other options refer to different means of illumination or operation. Manually would imply that the sign requires human intervention to illuminate or blank out, which is not the case with blank-out signs designed for automated traffic management. Externally suggests an outside light source is used, which does not apply here, and automatically refers to an activation without human input, but that does not specify the method of illumination. Internal illumination is the defining factor for blank-out signs, making it the correct choice.