Milestones are installed to provide reference points along the road. This can be used to reassure travellers that the proper path is being followed, and to indicate either distance travelled or the remaining distance to a destination. Such references are also used by maintenance engineers and emergency services to direct them to specific points where their presence is required. This term is sometimes used to denote a location on a road even if no physical sign is present. This is useful for accident reporting and other record keeping (e.g., "an accident occurred at the 13 mile mark" even if the road is only marked with a stone once every 10 miles).
Milestones (Latin: Miliarium) were originally stone obelisks – made from granite, marble, or whatever local stone was available – and later concrete posts. They were widely used by Roman Empire road builders and were an important part of any Roman road network: the distance travelled per day was only a few miles in some cases. Many Roman milestones only record the name of the reigning emperor without giving any placenames or distances. The first Roman milestones appeared on the Appian way. At the centre of Rome, the "Golden Milestone" was erected to mark the presumed centre of the empire: this milestone has since been lost. The Golden Milestone inspired the Zero Milestone in Washington, D.C., intended as the point from which all road distances in the United States should be reckoned. Odometers were used to measure the Roman milestone spacing, most likely based on Ancient Greek Technology.
A mile-marker monument, the Milion, was erected in the early 4th century AD in Constantinople. It served as the starting point for measurement of distances for all the roads leading to the cities of the Byzantine Empire, and had the same function as the Milliarium Aureum of Ancient Rome. The Milion survived intact until at least the late 15th century. Its fragments were discovered again in the late 1960s. A fragment is re-erected as a pillar.
In Europe, the distance measured typically starts at specified point within a city or town, as many roads were named for the towns at either end. For example, in London, United Kingdom, a plaque near the Eleanor cross at Charing Cross is the reference point from which distances from London to other towns and cities are measured. In the UK, milestones are especially associated with former Turnpike roads.
The British built many milestones on the island of Malta. They consisted of large slabs of local hard rock and they were engraved with the distance to or from a particular location. Many of these were defaced in World War II to disorientate forces in a potential invasion. Despite this, a very small number of milestones still exist undefaced, and one of these is now in the Malta at War Museum.