[Talk-GB] OSM Road Classification - Trunk/Primary

Barry McGuire barry86m at gmail.com
Tue Sep 14 16:50:37 UTC 2021

It's strange that the highway=primary tag is used for non-primary routes,
and it seems like an historical accident caused by people assuming trunk
roads and primary routes are the same thing, when they're not (usually).
Trunk roads form part of the strategic road network and are managed by a
highways authority (e.g. "National Highways" formerly Highways England, and
their relevant counterparts in Wales and Scotland). Primary routes are
selected routes between primary destinations. Other A roads are roads
connecting important (although not necessarily primary) destinations.

The Scottish OSM maps already have their strategic routes as trunk, with
all other A roads as primary, which more closely follows OSM terminology.

In Wales the vast majority of their primary routes are also strategic
(trunk) routes, but not all of them.

In England there is a proliferation of roads at the OSM 'trunk' level which
isn't seen in most other countries, especially not in town and city
centres. Many of the roads are currently tagged as 'trunk' because they're
primary routes but they're single carriageway, when OSM usage indicates
'trunk' roads are 'high performance or high importance roads that don't
meet the requirement for a motorway'. This implies they should be mostly
dual carriageway and high importance, yet fall short of motorway
classification (e.g. A1, A11, A12, A14, A19, A2, A20, A23, A27, A30, A31,
A34, A35, A38, A4, A47, A66). This further implies that using the
'strategic route' designation for the 'trunk' tag would be more appropriate
than using the 'primary route' designation.

Proposal to align UK road types with OSM types as follows:
OSM Motorway = UK Motorway
OSM Trunk = UK strategic routes, known as 'trunk' routes (Currently UK
primary A roads)
OSM Primary = All other A roads (Currently UK non-primary A roads)
OSM Secondary = B roads

Strategic route designations can be found by using data released under
OGLv3, just like the NaPTAN data.

There are a few edge cases to consider:-
* B5093 Mosley Road, Fallowfield, Manchester is designated as a primary
route and is also a B road. Resolved by changing to strategic = trunk.
* B6261 at Shap, Cumbria linking A6 to M6 is designated as a primary route
and is also a B road. Resolved by changing to strategic = trunk.
* A505 and A602 between Stevenage and Luton are not primary routes despite
linking two primary destinations. The proposal would place this road on the
same level as all other A roads, by abolishing the distinction between
'primary' and 'non-primary' A roads on OSM.

England-Scotland border (West to East) - 2 changes at the border (A7 and
* M6 Motorway
* A7 primary route (Scotland strategic, England non-strategic)
* A68 primary route (Scotland strategic, England non-strategic)
* A697 primary route (Scotland non-strategic, England non-strategic)
* A1 primary route (Scotland strategic, England strategic)

England-Wales border (North to South) - 1 change at the border (A465)
* A494 primary (Wales strategic, England strategic)
* A55 primary (Wales strategic, England strategic)
* A483 primary (Wales strategic, England strategic)
* A535 primary (Wales non-strategic, England non-strategic)
* A5 primary (Wales strategic, England strategic)
* A483 primary (Wales strategic, England strategic)
* A458 primary (Wales strategic, England strategic)
* A44  primary (Wales non-strategic, England non-strategic)
* A438 primary (Wales non-strategic, England non-strategic)
* A465 primary (Wales strategic, England non-strategic)
* A40 primary (Wales strategic, England strategic)
* A48 primary (Wales non-strategic, England non-strategic)
* M48 motorway
* M4 motorway

Changes of designation at borders are not unusual, so I'm not concerned
about that.

The only real downsides to this proposal are the work involved (which
shouldn't really take too long) and the loss of information about which
roads are designed as primary - which is a bit of a false distinction in
any case as primary routes do not indicate the actual standard or
importance of the road, just that they link two 'primary locations'. Note
that 'primary' locations do not mean they they're actually important
locations, just that they're useful for navigation purposes (e.g. Scotch
Corner as a junction, and Stevenage for itself and the towns around it).
The use of those locations for navigation purposes made sense in the 60s
but with satnavs etc the standard and relative importance of the road
should take priority on maps.

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