[OSM-talk] Speed tagging

[Jon Bright]

Alex Mauer wrote:
>>
>> Way 1, W->X maxspeed=50
>> Way 2, X->Y maxspeed:hgv:a=30;maxspeed:a=50;maxspeed:b=50
>> Way 3, Y->Z maxspeed:a=30;maxspeed:b=50
> 
> I would use "left" and right" instead of a/b, or "opposite" instead of b.

I'm going with

Way 1, W->X maxspeed=50
Way 2, X->Y hgv:maxspeed:opposite=30;maxspeed=50
Way 3, Y->Z maxspeed:opposite=30;maxspeed=50

(The segments/ways are pointing uphill, the ways in my mail are the 
"wrong" way round)

Since we already have a hgv tag elsewhere, I'll leave that hairball for 
someone else and use it.

I picked "opposite" over "left"/"right" because "opposite" only needs a 
route planner to know in what direction it wants to travel over the 
segment.  "left"/"right" needs the planner to know its direction over 
the segment *and* to know that you drive on the left/right-hand side of 
the road at this lon+lat...

I'd imagine a route planner doing something like:

  - If travelling in the direction opposite to the segment direction, 
search for "<vehicle class>:maxspeed:opposite"
  - If travelling in the direction opposite to the segment direction and 
the above tag wasn't present, search for "maxspeed:opposite"
  - If travelling in the segment direction, or if the above tags weren't 
present, search for "<vehicle class>:maxspeed"
  - If no tags yet found, search for "maxspeed"
  - If still nothing found, take some sensible default based on the 
highway class. ("sensible" depends on national boundaries, but this is 
the route planner's problem, not ours)

--
Jon