This is one of the old 'Quaking Bridge' road signs. 

Quaking Bridge is a historic structure located in the city of Oxford, England. Its name is believed to derive from the sensation it once produced when people walked over it, as the bridge would tremble or "quake" slightly due to its construction which would originally be made of timber. 

There has been a bridge at this location from at least the late 13th century. Quaking bridge was first mentioned in 1297 but is probably much older. It crossed the Castle Mill Stream immediately west of Oxford Castle, on a road that connected Oxford and St Thomas's parish before the castle existed.