[OSM-talk] Divided roads proposal

[Lester Caine]

Steve Bennett wrote:
> On Mon, Dec 7, 2009 at 12:30 AM, Lester Caine <lester at lsces.co.uk 
> <mailto:lester at lsces.co.uk>> wrote:
> 
>     I think my only problem with 'divided' is "At what point do you
>     apply it?" The
>     samples being shown are quite clearly - on the whole - dual carriageway
>     structures. 
> 
> (Just on terminology, I'm used to "dual carriageway" only being applied 
> to motorways, but Wikipedia says it technically applies to any road. 
> We'll go with that, then, ok.)

MANY major routes in the UK are trunk roads and most routes around cities will 
have one way elements that split and join at different points. YES it is the A?? 
and a single name, but the structure can only be mapped as a dual carriage way. 
Which then takes us to some of the 'green way' areas where cars go down either 
side of a grass verge. A simple 'divided' in your tag, but the separation may 
grow from nothing to several meters. At what point do you change from 'divided' 
to separate ways, which then begs the question - why have divided if it's just a 
  shorthand for two ways with opposite directions.

> By "are clearly dual carriageway structures", I take it you're 
> distinguishing between roads which have a shortish traffic island of 
> some type, versus those which are divided for a long stretch. Is this 
> important?
> 
>     Example 10 clearly has a more complex structure than can be mapped
>     by showing a 'divided' tag, since there is no access to the joining
>     road from
>     the other carriageway?
> 
> Have you read the proposal?
> http://wiki.openstreetmap.org/wiki/Proposed_features/Divided_road

You need to justify the real need for it. I'll continue to map the actual 
structure, and add the additional ways for the related footpaths. I don't see 
the need for this shorthand for many of the cases you are trying to make?

> It covers *exactly* this case. In my proposal (ie, not Vovanium's, which 
> cohabits the page),  when there's a junction (like the road entering 
> from SW), by default that road can't cross the median - there is no gap. 
> You can see how this looks in my mocked up Halcyon image.

I think what YOU are missing is that in most cases where there are traffic 
islands which add one way sections of way, they ARE mapped. Around here there 
was an attempted to remove some of them, but that has been rolled back, so where 
a road splits, the correct direction ways are added. Routing does not then need 
to run through lots of additional tags to find if it can then do a maneuver ...

>     Example 6 has some quite complex slip roads that really need
>     isolated ways for
>     the main carriageway. Trying to ADD tags to supplement a simple
>     'divided' tag to
>     explain the slips on and off at the end is handled much easier with
>     a simple
>     dual carriage way? 
> 
> Do slip ways need to be modelled at all? At the moment, they're not, as 
> far as I have seen. Essentially what is going on at that intersection is 
> very straight forward: divided road meets (temporarily) divided road, 
> and all turns are possible. Currently, I don't think many people would 
> map the N/S as divided (ie, two ways). With this proposal, you could do 
> so, without creating a mess.

I think it is essential that slipways are mapped. ESPECIALLY when one is trying 
to add the right routing instructions. TomTom has started showing motorway and 
major road slipway details properly. You need to know when to get to an inside 
lane and take a slip road PRIOR to the actual junction. These are no different 
to the island details approaching a roundabout, so trying to 'save time' by not 
actually adding quite important detail does seem wrong? A tag saying you should 
have taken the slip 10 mts before this junction is not sensible.

>     And many of the other examples need the same end cases. So at
>     what level does a simple 'divided' tag actually work in practice?
>     However
> 
> IMHO, the divided tag is well suited to all cases except 6, 7, and 9. 
> Number 2 in particular is a perfect example. Worth tagging, not worth 
> splitting the road in two for.
> 
>     'double white lines' on a single carriage way road IS a divider that
>     needs tagging?
> 
> Yeah, I deliberated over whether or not to include that. What do you 
> think - same proposal, or separate? Double white lines really aren't a 
> "divider", they're a restriction on overtaking.

Example 3 is no more than a wide 'double line' road marking. SO is it a 
'divided' or is it simply a road marking? The problem with the proposal is that 
it does not have any indication on when it should be used ... or when the more 
detailed current methods are actually more appropriate?

-- 
Lester Caine - G8HFL
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