From Roader's Digest: The SABRE Wiki
A spur is a road (often a relatively short one) that turns off to one side from another (typically more important) road.
On SABRE the term spur is mostly used in the more specific sense of a spur which shares the road number of the road from which it turns off, giving that numbered road three (or more) endpoints.
The original classification of roads was designed more to identification for funding and maintenance than navigation purposes, and consequently spurs did not originally exist. However, as road numbers became to feature more prominently on signs from the 1930s onward, it was decided that making short links spurs of longer roads would aid in navigation.
Barring one or two earlier exceptions, spurs as we know them today were created in 1933 by David Therrel, who had become responsible for road numbering in the Southern Engineering Division. Given the task of revising road numbers after construction of the Basingstoke Bypass, he decided to not use the then current practice of allocating a 4 digit number to the former A30 between the eastern end of the bypass and Hackwood Road, but simply numbered it as a spur of the A339. After mentioning this when asked for his comments on improving road signing, the MOT decided to take a country wide look at rationalising the numbering of roads, which led to the 1935 Road numbering revision, creating many spurs in the process.
Today, probably the most common cause of a spur is of a minor road joining a now bypassed road, and splitting in two directions to cover all of the old route. For example, the A559 once turned off to the north from the main A556 near Northwich; when the A556 was re-routed along a new bypass to the south of the town, the bypassed length of A556 became A559, creating a three-arm junction at which all arms are A559.
Motorway spurs are listed here.
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