[OSM-talk] Relations not always brilliant

[Richard Fairhurst]

David Earl wrote:

>>     In the UK, road numbers are unique (apart from about three cases
>>     where local councils have cocked up, e.g. the B4027) and no  
>> road can
>>     have more than one ref.
>
> Not true - the A11 and A14 share about 10 miles of dual carriageway  
> around the north of Newmarket, for example.

It's absolutely true. That bit's the A14. This Highways Agency  
document, for example, refers to the stretch of road in question as  
solely the A14:

     http://www.highways.gov.uk/roads/15200.aspx

The fact that traffic "following the A11" needs to use it is pretty  
much immaterial - traffic following the A34 from Winchester to  
Manchester, for example, has to use the M40 from Bicester to the M42,  
and no-one's suggesting that the M40 is also the A34 (if it is, I can  
cycle on it ;) ). No, it's the A14 leading to the A11, and will  
almost certainly be signposted as such - "A14 (A11)", or on more  
recent signs, on separate lines like this:

A14
Bury St Edmunds 15
Felixstowe 87
(A11)
Norwich 98

There are thousands of stretches of road like this across Britain,  
but in all cases they only have one official number (very occasional  
signage errors notwithstanding).

>> The relation doesn't give any info over and
>>     above that in the standard 'ref' tags - it just increases  
>> complexity
>>     for both editing and processing.
>
> It links the pieces together, which you would have to infer  
> otherwise from the ref. That's not to say the ref shouldn't be on  
> the highway as well.

But if you can unambiguously infer it, you shouldn't need to  
explicitly tag it.

Having duplication also makes it too easy for discrepancies to arise  
- what if a newbie changes the ref in the way tag (obvious), but  
doesn't update the relation membership (less obvious)?

cheers
Richard