Does the tertiary highway classification correspond to 'C' roads in the UK?

amurski · 2022-10-18T15:43:00.954Z

I recently found some inconsistency in the usage of tertiary highways and attempted to fix them based on local authority classification. However, I’m not sure if this is the correct approach since I’ve noticed other highways have been downgraded on the basis that they were ‘incorrectly’ mapped. It’s not clear what the correct approach is. I would tend towards using the classification and ensuring that the lane count is correct but I’d like to hear counter arguments before applying this any further.

Richard · 2022-10-18T17:40:26.015Z

No, it absolutely doesn’t.

Generally a highway=tertiary in the UK is one which has a significant through traffic function, and is engineered accordingly, but isn’t a B road. As a rule of thumb, expect two lanes, probably a centreline, and priority at most junctions (other than with A/B roads, obviously). An HGV should usually be able to proceed down a highway=tertiary without incident (except if there’s a weight/height limit).

Unlike the higher values, this doesn’t correspond clearly to a UK road classification. This appears to offend some people’s sense of order and therefore there have been several well-intentioned but ultimately quite damaging attempts to “fix” this. Please don’t. The classifications have been broadly correct for many years and do not need fixing.

As a side issue, there is no consistency between local authorities in the UK as to how they use their C/D/E/F/U/whatever classifications. So any attempt to reclassify “C roads” as highway=tertiary would instantly introduce inconsistency.

Here are some roads that are definitely highway=tertiary:

Ashendon - Cuddington Road © Jackie Harman :: Geograph Britain and Ireland
The Wisp towards Withcote © Andrew Tatlow cc-by-sa/2.0 :: Geograph Britain and Ireland
Astrop Cottages © Jonathan Billinger :: Geograph Britain and Ireland
2011 : Old Wells Road © Maurice Pullin :: Geograph Britain and Ireland
Luppitt: the road to Smeatharpe © Martin Bodman :: Geograph Britain and Ireland
Cribarth © Richard Webb :: Geograph Britain and Ireland

SK53 · 2022-10-18T18:47:17.297Z

@Richard covers the main points.

There have been several discussions on the talk-gb mailing list (which continues to be the best place to ask UK-specific mapping questions) on this subject over the years. The most recent one, I believe, was this. Most such discussions stress that usage of C-road is not consistent between authorities. Where people have used it as a basis this has generally resulted in a number of roads being inappropriately tagged as tertiary, which may have ramifications of various routing applications (HGVs on narrow roads; cycling & walking apps avoiding little used roads).

If you do wish to record the classification used by the local highway authority then the general view of the UK community was to use a modified ref key, such as highway_authority_ref (see the wikipage). This allows access to the internal classification for data consumers interested in such data, and avoids spurious unsigned information cluttering up maps, and voice directions by satnavs.

amurski · 2022-10-18T21:17:59.348Z

OK so it looks like I had found partially mistagged tertiary rather than the reverse and I will need to revert some stuff. Shouldn’t be a problem though.

Richard · 2022-10-19T08:13:14.749Z

Thanks!

I suspect you might have encountered a bunch of roads that were retagged to tertiary by Sam888 around a month ago. For the most part these had been unclassified for many years (sometimes 10+) and should have remained so.