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52 Wayfinding and Signing Guidelines for Airport Terminals and Landside critical stakeholders (such as law enforcement, security, transportation, and public safety) can be used to establish these agreed upon criteria. Protocols or hierarchy for prioritizing messages and determining which messages are to be displayed. Geographic area over which the information is to be displayed, to be determined in coopera- tion with public safety and security agencies. Identification of the circumstances under which transportation-related messages, such as lane closures, fog alerts, detours, or other messages that may be needed because of dangerous travel conditions in the immediate vicinity, would preempt emergency or security alert messages. The criteria that would cause the discontinuation of use of the CMS if the emergency or secu- rity alert message creates an adverse traffic impact such as queues, markedly slowing traffic, etc. Methodology for developing and displaying messages that are appropriate for CMS display including, but not limited to, standard message sets. Agencies should follow the recommended national CMS practices related to the development, use of text, manner in which messages should be displayed, human factors related to understandability of the messages, and how CMS are operated. 3.7 Sign Maintenance In 2008, FHWA passed rules concerning minimum sign retroreflectivity values. These are listed in Table 2A-3 of the MUTCD. These standards require a sign maintenance plan that ensures that minimum levels of retroreflectivity are maintained to provide adequate visibility. The following five different methods of regular inspection of sign retroreflectivity are allowed: Visual nighttime inspections, Measured retroreflectivity, Expected life, Blanket replacement, and Control signs. Sign retroreflectivity can be incorporated into an overall asset management plan. Expected life of sign materials and structures can be included in the plan to expedite routine replacement and maintenance. The retroreflective material on a sign can be expected to last 712 years depending on the type of material selected and its sun exposure. Ground-mounted signs may need to be cleaned annually to remove dirt and mud, particularly those in splash zones and near areas where vehicle idling may produce soot build-up. Annual daytime and nighttime visual drive-by inspections are recommended as part of a maintenance schedule. Sign supports, crash cushions, and guardrails may need routine maintenance and inspection as well depending on the system used. Manufacturer's recommendations should be followed to check for rust, loose bolts, etc.