[Tagging] Counting lanes: include merging or turn lanes?

[Alan Mintz]

At 2010-07-04 13:41, Alan Millar wrote:
>How are people using the "lanes" tag on motorways?  Do you count the
>on/off ramps that come and go?

Generally, no.


>On a lot of the freeways near me, they have two main driving lanes on
>each direction, so I set it up as a dual carriage way with one-way on
>each side, and mark each side as lanes=2.
>
>The question comes in for the on/off ramps.  There are sections where
>two intersections are somewhat close together, and so there is an on- 
>ramp, a long extended merging lane, then the next off ramp.
>Strictly speaking, there is a third lane for some distance between
>the the ramps, but it isn't a "through" lane.  There are only 2
>driving or through lanes in that section of freeway.
>
>See for example hwy 217 between Denney and Allen at:
>
>http://www.openstreetmap.org/? 
>lat=45.473345&lon=-122.786743&zoom=18&layers=B000FTT

I generally, form the intersection of the ramp with the main roadway at the 
point where the solid line starts or ends - the first place you can legally 
merge from an onramp or the last place to an offramp. In this case, this 
leaves too short a section of 3-lane main roadway to bother breaking it 
just in order to tag that short section with the extra lane. I see the 
lanes tag being useful primarily in determining likely speed possible in 
making routing decisions, other factors being equal. It could also be 
useful in rendering. Neither would seem to sway me to add the extra lane 
for such a short distance.

I-10 in this area 
http://www.openstreetmap.org/?lat=34.03753&lon=-118.280316&zoom=18&layers=B000FTF 
might end up being an exception in some places (I didn't look closely, but 
it comes to mind).


>I have only been counting the driving or through lanes, so I have
>tagged this as lanes=2, and just ignored the short extent of the
>extra merging lane.

Agreed.


>Likewise, there are suburban streets that have an extra turn lane
>just before an intersection, like Murray Blvd at:
>
>http://www.openstreetmap.org/? 
>lat=45.4814&lon=-122.8261&zoom=18&layers=B000FTTT
>
>As one big street with two driving lanes each direction, I've tagged
>it as lanes=4 and ignored the short extent of the extra turn lane.

Because that center lane is present between intersections as a center turn 
lane, I tag lanes=5. This, to distinguish it from roads where there are 
only 4 lanes (2 in each direction) for the stretches between intersections, 
since left turns without that center turn lane cause more congestion (again 
thinking of speed estimation).


>And one more variation is the suburban boulevard with one center
>lane, only allowed to be used for turning/merging in and out of
>driveways.  I consider this like the freeway merging lanes; I've just
>been counting the driving lanes and not the turning lane also.  See
>Canyon Road at:
>
>http://www.openstreetmap.org/? 
>lat=45.489479&lon=-122.799664&zoom=18&layers=B000FTTT

This looked the same to me as the one above. I tag lanes=5.

--
Alan Mintz <Alan_Mintz+OSM at Earthlink.net>