Specifies formal procedures and two, alternative, methods for determining the priority of on-board messages presented to drivers of road vehicles by transport information and control systems (TICS) and other systems. Applies to the whole range of TICS-n-vehicle messages, including traveller information, navigation, travel and traffic advisories, "yellow pages" information, warnings, systems status, emergency calling system information, and electronic toll/fee collection, as well as to messages from non-TICS sources such as telephone, warnings and telltales.