[OSM-talk] route=road - What's that all about then?

[Andy Street]

[reply-to set to talk-gb so we don't bore the rest of the world!]

On Sun, 24 Aug 2014 12:23:47 +0100
"Dave F." <davefox at madasafish.com> wrote:

> On 24/08/2014 00:10, Andy Street wrote:
> > That's not strictly true, we do multiplex routes but individual 
> > sections of road are only ever referred to by a single route number 
> > (usually the most significant route being carried by the road).
> 
> Unsure what you mean by 'multiplex'. Do you have an example?

Essentially it is where two or more separate routes join together and
run along a single physical section of road before diverging and
continuing on their separate routes. In the UK when this happens only a
single numbering scheme is used (normally the more important route).

To give you a concrete example of this, consider the A272[1] which runs
between the A267 in East Sussex[2] and the A30 in Hampshire[3]. Working
backwards from the western end, the route runs south-west until it meets
the A34 where it multiplexes until the roundabout at junction 9 of the
M3. There is a short non-multiplexed section heading south (Spitfire
Link) before multiplexing with the A31 heading east. After about a mile
the two routes diverge and the A272 heads off cross country towards
Petersfield.

Since major UK road numbers are intended to be unique with the first
digit signifying the zone that the road starts in[4] it is clear that
the most westerly sections described above are a continuation of the
route that started in zone 2 rather than separate individual roads.

[1] http://www.sabre-roads.org.uk/wiki/index.php?title=A272
[2] https://www.openstreetmap.org/node/1685712401
[3] https://www.openstreetmap.org/node/683002
[4] https://en.wikipedia.org/wiki/Great_Britain_road_numbering_scheme

-- 
Regards,

Andy Street