# [Tagging] Use cases: where would a count of all lanes be useful?

Anthony osm at inbox.org
Tue Sep 20 00:42:04 BST 2011

```On Mon, Sep 19, 2011 at 7:33 PM, Stephen Hope <slhope at gmail.com> wrote:
> On 20 September 2011 00:39, Gerhard Hermanns <hermanns at ptt.uni-due.de> wrote:
>>
>> Traffic simulation (e.g. by cellular automata) is a use case for this. At
>> crossroads it can make a huge difference for traffic engineering to know if
>> a (turn) lane can contain 3 or 30 vehicles.
>> To determine the "capacity" of a turn lane one would need information about
>> its length - and one would get this easily if start and end point of the
>> lane are known. Otherwise one would have to tag something like
>> "turn_lane_left_1=xx" and "turn_lane_left_2=yy" (indicating length or
>> capacity or whatever) in case of two turn lanes with different lengths.
>
> I'm not sure I would say "easily".  What you're looking for here is
> the number of cars that can be stored in the turning lane without
> blocking flow in the through lanes, correct?  Where do you mark the
> extra lane width as starting?  Where the lane first splits off? Where
> the centre of the lane splits off?  Where the lane is wide enough for
> a car to fit?  Where the line markings for a normal lane start?  In
> some places, these are all close enough that it wouldn't matter much
> for your purposes, but there are places near me where these different
> places would make a big difference in car count - 30 - 50m is not
> unusual, and some are more.
>
> For a road splitting off from the main road, we would mark the split
> in the centre of the road, so for consistency should lane count
> changes be done the same way?

I'd say the lane count should be changed when the lane is completely
there.  So lanes=2 becomes lanes=3 when there are 3 full lanes.  If
you wanted you could even split off a way between lanes=2 and lanes=3
and call it lanes=2.5.  Renderers could then interpolate in this zone.

On Mon, Sep 19, 2011 at 3:45 PM, Nathan Edgars II <neroute2 at gmail.com> wrote:
> Ah, http://wiki.openstreetmap.org/wiki/Relations/Proposed/turn_lanes . Would
> it be possible to modify this to allow a node for the beginning of the lane
> rather than its length?

If you split the way, to change the lanes=*, then you necessarily have
a node at the beginning of the lane.  And it's obvious what the node
represents, rather than having to dig through relations to figure it
out.  A node in the middle of a way with no tags of its own, and no
split in the way, is apt to get inadvertently moved or deleted.

```