Why, on freeway overp construction, are pieces of whiteboard clamped to the uprights facing traffic?

On my morning commute, I drive through freeway construction where an overp is being built. On either side of the freeway, attached to the upright steel reinforcements, are pieces of white drywall. There are even relatively low-wattage lamps illuminating these temporary-looking panels. There are no pieces on the sides where we would see them in our rearview mirrors, and no other panels of drywall. The traffic in the opposite direction has the same setup. I have to think it was done on purpose. Why are these panels there?