If any directional or traffic control sign installed or maintained by the department within the right-of-way of a state highway is defaced with graffiti in a manner which interferes with the ability of motorists to comprehend the information which the sign was intended to convey, thereby posing a danger to the public, the department shall, as soon as reasonably possible after learning of the graffiti, either remove the graffiti and treat the sign with an anti-graffiti substance or material, or replace the sign, whichever is more practical and economical.