[mkgmap-dev] Turn restrictions with role=via ways

Tue Mar 18 14:53:27 GMT 2014

Hi Steve, Gerd,

On Tue, Mar 18, 2014 at 11:16:53AM +0000, Steve Ratcliffe wrote:
>Imagine there are two two roads between A and D; ABCD and AD. You can 
>travel AB and ABC, but there is a turn restriction from C to D. So
>the 'real' turn restriction is BCD, but they write it as ABCD, perhaps
>just so that the algorithm doesn't waste time exploring the wrong
>path if it is at A and needing to get to D.
>
>I've no idea how well that relates to what the OSM tags mean.

I suspect that in the example that I posted, the mapper wanted to map 
for the renderer (JOSM in this case).

If he used only from,via,to ways in the turn restriction, the "no turn 
allowed" traffic sign would show up in the middle of the junction (of 
two dual-oneway roads). It looks nicer in JOSM if the sign is before the 
junction, like in the real life.

This is of course bogus, just like it is redundant to add a turn 
restriction that prohibits turning to an oneway road against the oneway 
direction. If you want to map traffic signs, there is an established 
tagging scheme for that in Finland.

I did not contact the mapper yet to confirm if this is the case.

Anyway, the problem is that the U-turn restriction is ABCD 
(A=from,B=via,C=via,D=to) while BCD (B=from,C=via,D=to) would suffice.

AFAIU, other turn restrictions than no_u_turn can always be mapped 
without via ways (using via nodes). Do you agree? Could we automatically 
convert such turn restrictions to ones that use via nodes? The 
translation would do the following:

1. keep the role=to way as is
2. add role=via node at the junction of the role=via way and the role=to 
way
3. rename the role=via way from step 2 to role=from way
4. discard any other role=via ways, and all role=from ways

Does this sound correct to you?

Supporting role=via ways for no_u_turn would be a bigger task (requiring 
knowledge of the Garmin format).

	Marko