If wire rope guardrails are used, what must also be done? a. no further action is required b. they must be flagged with highly visible materials at least every six feet c. they must be flagged with highly visible materials at least every twelve feet d. signs must be posted

In the proper implementation of Wire rope guardrail it is a standard industry practice to use highly visible material every six feet, as this distance is understood to be ideal for the best visibilty to usability ratio. This is important as the guardrails that are installed need to be properly visible so that accidents can be avoided.