All warning signs used at night should be fully illuminated or what?

The correct answer is that all warning signs used at night should be retroflective. Retroreflective materials are designed to reflect light back to its source, making them visible to drivers even in low-light conditions. This characteristic is especially critical for warning signs, as they need to convey important information to drivers at night or during inclement weather.

By using retroreflective materials, these signs enhance visibility significantly when illuminated by vehicle headlights. This is vital for ensuring the safety of both drivers and pedestrians, as it allows for better reaction times and awareness of potential hazards in the roadway environment.

While reflective materials, mentioned in one of the options, do provide a level of visibility, they often do not reflect light as effectively as retroreflective materials do. Hence, retroflective is the specific term used to ensure maximum visibility of warning signs during nighttime conditions.