It is worth noting that this signage (albeit not the content) is standardised throughout Europe: upon crossing a border or leaving an international zone (e.g. airport, ferry port) signs such as this will be provided indicating rules for towns/cities, open roads, designated roads, motorways and other pertinent requirements. For other signs, refer to any number of links (I have advised this to my US in laws when they visit, as their signage is much less internationalised). https://www.autoeurope.com/roadsigns/