A milestone or kilometre sign is one of a series of numbered markers placed along a road or boundary at regular intervals, typically at the side of the road or in a median. Milestones are constructed to reassure travellers that the proper path is being followed, and to indicate either distance travelled or the remaining distance to a destination. They are alternatively known as mile markers, mileposts, or mile posts (sometimes abbreviated MPs), notably in the United States.
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 13.45 km mark" even if the road is only marked with a stone once every 10 km).
Milestones 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, in which when the distance travelled per day was only a few miles in some cases. The first Roman milestones appeared on the Appian way. At the centre of Rome, the "Golden Milestone" was erected to mark the metaphorical 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.
Milestones on Indian highways typically have white backgrounds with yellow tops (on national highways) or green tops (on state highways). The names of cities and distances are painted in black. The names of the nearest towns and cities are written along with distance in kilometers. On undivided highways, both sides of the milestones are used, telling the distance to the nearest cities in each direction. The highway number is written on the head of the milestone. The sum of the distances of two nearest cities in each direction from the milestone is listed on the side.
In 1845, the Railways Clauses Consolidation Act was passed, compelling UK railway companies to provide their passengers with a means of determining the distance traveled (fares were set by distance at this time). Section 94 states:
"The company shall cause the length of the railway to be measured, and milestones, posts, or other conspicuous objects to be set up and maintained along the whole line thereof, at the distance of one quarter of a mile from each other, with numbers or marks inscribed thereon denoting such distances.
Similar laws were passed in other countries. On the modern railway, these historical markers are still used as infrastructure reference points. At many points, the distances shown on the markers are based upon points no longer on the network – for example, distances measured via a closed line or from a junction which has subsequently been moved. Whole mileposts are usually supplemented by half and quarter posts. Structure signs often include the mileage to a fair degree of precision; in the UK, a chain is the usual accuracy. In the U.S. and Canada, miles are "decimalized", so that, for example, there may be a "milepost 4.83".
In metricated areas, the equivalent is the "pointe kilometric".
Surveyors place milestones to mark the boundaries between the jurisdictions separated by borders. A series of such boundary markers exists at one mile intervals along the borders of the District of Columbia in the United States.