[OSM-newbies] divided but not seperated

[Andrew Errington]

On Mon, August 24, 2009 10:52, Robert Helvie wrote:
> The city I am mapping has recently started to divide certain of the
> larger, more major, roads that I originally mapped as single ways. Now,
> all they are really doing is putting a concrete strip down the center with
> shrubs, or even in some cases, just installing metal "bollards" down the
> centerline. I assume this is to prevent people from driving across on
> cross streets or to stop people from making crazy u-turns.
<snip>
I have the same issue, and I was going to ask about it.  I had a short
discussion offlist, and it seems I am not too crazy.  I have also taken
some photos of the roads in question, so I could put them up somewhere to
illustrate the discussion (where?  I am new to this.)

In my opinion the road should remain as a single way, not least because it
is terribly difficult to draw parallel lines, but also because the
dividers only divide long sections of road and there are breaks in the
divider at most of the intersection (i.e. there is no logical difference
between a piece of road with and without a divider).  The roads in my town
are divided with either a small concrete curb, maybe a foot tall, and a
foot wide; a row of plastic bollards, maybe two feet high, the diameter of
a coke can, spaced about every foot; or Armco barrier (the pressed steel
stuff [1]).

The barriers stop and start at various points along the length of the road
to allow breaks for (some) intersections.  There are also very short
sections *at* intersections (usually plastic bollards) to *prevent* you
from going across (i.e. to force a turn in only one direction).

My suggestion is a "divided=*" tag, where 'divided=yes' implies some
arbitrary *thin* barrier meaning you cannot drive across or do a U-turn. 
For more detail use 'divided=plastic_bollards' 'divided=concrete_low'
'divided=concrete_high'  'divided=armco'.  I am aware that Armco is a
trademark, and some of these other things might have more "proper" names
(is there a civil engineer in the house?)

Here is a road network (+ is a node, - is a way)
             B
A +----------+------------+ C
             |
             |
             +
             D
Fig 1

An autorouter coming from D can assume it can turn left or right at B
unless the oneway tag is set on AB BC or AC.

Then the local authority divides AC.  It leaves a gap at B.  So you
resurvey and add points E and F to represent the end of the barrier
(conveniently in my example the barrier starts at A and C.)

           E B F
A +========+-+-+=========+ C
             |
             |
             +
             D
Fig 2

So, AE and FC has 'divided=yes' and EF does not.  A renderer could draw a
thin line in the centre of AE and FC but not in EF, thus making the
restriction visible (shown in my diagram with =).  An autorouter has no
problem because the way EF through B is not divided (you can see I am
making things up).

If the authority divides AC:

             B
A +==========+===========+ C
             |
             |
             +
             D
Fig 3

Or just puts a short section (EF) in, this would prevent a turn (in Korea
this would force a right turn)

           E B F
A +--------+=+=+---------+ C
             |
             |
             +
             D
Fig 4

Then an autorouter would have to detect the divided condition at B and
allow only a right turn (or left in the UK).

Also, in this case:

                   H
                   +
                   |
             B   E | G
A +----------+---+=+=+----+ C
             |     F
             |
             +
             D
Fig 5

EG is divided=yes, so an autorouter must determine that a left turn (in
Korea) from AF to FH is not possible, but a right turn (in Korea) from CF
to FH is.

In some ways drawing two separate oneway ways would be more explicit, but
it would be quite tedious for the cases where a way has many short
sections of barrier (which is my situation).  In fig. 4 you would go from
a 4 lane way to two two-lane oneway ways (with a road BD leading off one
of them) and then back to a 4 lane way...

I guess if there were tools to aid with drawing parallel lines then it
would be easier (I am using JOSM), and if we properly recorded the width
of each road then renderers could render two 2-lane ways of 6m each in the
same width as one 4-lane way of 12m width.  Above, I said that the
dividers were thin.  Perhaps a rule of thumb would be if the ways are
separated by more than, say, 3 metres, then divided=* should not be used
and ways should be drawn separately.

This idea *could* be extended to apply to motorways and dual-carriageways,
but to be honest I think it's better that these are explicitly entered as
oneway ways in each direction (and it would break the rule of thumb I just
made up).

Sorry for the rambling post.

Andrew

[1]http://www.armcorsp.co.za/index.php?page=guardrail