What characteristic must signs and devices used for directing traffic at night have?

Signs and devices used for directing traffic at night must be retroreflective to ensure they are visible and easily recognizable to drivers in low light conditions. Retroreflectivity allows light from vehicle headlights to be reflected back towards the source, making the signs more noticeable when illuminated by oncoming traffic. This characteristic is crucial for maintaining safety in work zones during nighttime, as it enhances visibility and comprehension of the instructions being conveyed.

While illuminated signs can also enhance visibility, retroreflectivity is a key requirement outlined in traffic control standards because it works effectively with existing vehicle headlights. This means that even without additional lighting, retroreflective materials provide a reliable means of communication for drivers navigating through work zones at night.