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

Paul Johnson baloo at ursamundi.org
Tue Jul 6 05:20:17 BST 2010

On Sun, 04 Jul 2010 19:21:36 -0700, Alan Mintz wrote:

> 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:
> 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/?
> 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
> Agreed.
>>Likewise, there are suburban streets that have an extra turn lane just
>>before an intersection, like Murray Blvd at:
>>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).

I tend to discount restricted, merging and turn lanes myself, and count 
restricted lanes as a seperate way as a stop-gap measure until lane 
restrictions are more or less sorted.  I wish I had a better tagging 
solution than that, since a lot of interchanges in Portland are behind 
blind curves in no-lane-change sections.

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