[Routing] [motorway_link] description of direction

[Jon Bright]

Marcus Wolschon wrote:
> 
> If you have multiple lanes that lead to different locations
> I suggest modelling them as multiple ways from the latest location
> where a sensible lane-change is possible.

Sure, and that's what I do.  But you want your directions somewhat 
before you get to the last location where lane-changing is possible. 
Quite often, there are signs indicating what lane you want to be in 1km 
or more before the last sensible lane-changing location.  At this point, 
the road is still very clearly one road.  You're legally, practically 
and easily able to change between the lanes.  So it should be one way, 
with (say) lanes=4.  Later, when it's split (at least for legal purposes 
- thick white lines, whatever), it's two or more ways.  But I want to 
know which lane to be in now, as the overhead gantry is offering me a 
choice.  So we need some sort of signpost recording system that lets me 
take the output of the nav system and correlate that with what I'm 
seeing on the overhead gantry.

Here's an example:

http://www.aidan.co.uk/md/LonM25J8Signs5404.jpg

The last safe/legal/practical change of lane after that sign is half a 
mile (~800m) later.  But (if need be) I want instruction about lane 
changes right now - or in fact 500m or so ago, so that the instruction 
is being reacted to just as this sign appears.

Hence the need for some kind of signpost-recording system.

Just for reference purposes, here's a commercial map provider's tagging 
of that same junction:

http://tinyurl.com/6nkdwm

-- 
Jon Bright
Silicon Circus Ltd.
http://www.siliconcircus.com