[Tagging] exit_to on motorway_junction

Simone Saviolo simone.saviolo at gmail.com
Mon Nov 19 09:51:30 GMT 2012

2012/11/18 Philip Barnes <phil at trigpoint.me.uk>

> On Sun, 2012-11-18 at 21:26 +0100, Colin Smale wrote:
> > Phil, there's a difference between routing calculation (which neither
> > knows nor cares about road names, numbers, signposts etc) and how the
> > result of the calculation is presented to the user. Then you need to
> > relate the nodes/edges in the routing graph back to the real world. The
> > value in this tag (as well as name, destination etc) is that the
> > navigation software can give instructions based on recognisable
> > landmarks instead of referring to way IDs. This distinguishes between
> > pure "routing" and "useful navigation".
> >
> Thats the point I am making, the most useful instruction a satnav can
> give is "leave the motorway a junction 4", or words to that effect.
> There is nothing more recognisable than the junction number.

In this motorway intersection [1], where there's no evident "exit lane",
the difference between "follow right", "follow right to the A8/A26
motorway", and "follow towards Gallarate, Malpensa, Milano" is crucial. Of
course we could have navigators ( != routers) that only tell us to go right
and left, but having they call roads by their name it's a great help. In
that case, there would be a large overhead sign with an arrow pointing
towards "Milano (A8)" and another one towards "Gravellona Toce (A26)", and
those would be the easiest instructions to follow, since you'd be already
looking at the sign anyway (even better than right/left).

In non-motorway situations this would be great instructions too. Suppose
you're driving along a long straight road, and there are two crossroads
close to each other (say, about 50 metres away). I would be hesitant to
pick one of them based only on an instruciton like "in 100 metres, turn
left" - an instruciton that is given as I'm driving, for example, at 70
km/h, which is about 20 m/s. However, if I was told "turn left to
ThatVillage", I would quickly match the instruction to a sign on the
ground, and I would safely and surely taking the correct turn.



[1] http://www.openstreetmap.org/?lat=45.69585&lon=8.54584&zoom=16&layers=M
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