Milestones have a very long and important history, from the days of the Romans until the 19th Century, when they were the direction signs of their day. They were also used for keeping coaches on schedule in the 18th and early 19th centuries, and formed the basis for postal charges until a standard postal rate was introduced in 1840.
From 1888, local authorities took over the responsibility for major roads from the turnpike trusts, and with it responsibility for the upkeep of milestones. This example, a triangular design (presumably in cast iron) by the City of Westminster from 1911, is relatively late, and is a rare survivor in London. It points from Hyde Park Corner to Hounslow - clearly an important place in 1911!