Countdown markers at exit from motorway

This sign means: ‘Countdown’ markers at exit from motorway (each bar represents 100 yards to the exit). It is an information sign. Information signs are all rectangular and tell you about the road and what is on it.

Official Highway Code caption: “‘Countdown’ markers at exit from motorway (each bar represents 100 yards to the exit). Green-backed markers may be used on primary routes and white-backed markers with black bars on other routes. At approaches to concealed level crossings white-backed markers with red bars may be used. Although these will be erected at equal distances the bars do not represent 100 yard intervals.”

Listed under “Rectangular signs” in The Highway Code: traffic signs (GOV.UK). For every sign in more detail, see the Department for Transport’s Know your traffic signs.