Explanation

"On a motorway, it's best to use a roadside emergency telephone so that the emergency services are able to locate you easily. The nearest telephone is indicated by an arrow on marker posts, which are placed every 100 metres along the edge of the hard shoulder. If you do use a mobile phone, the emergency services will want to know your exact location. Before you call, find out the number on the nearest marker post; this will tell the emergency services your exact location."