[Tagging] Permissive turn restrictions

Svavar Kjarrval svavar at kjarrval.is
Fri Sep 2 21:54:11 UTC 2016

On fim 1.sep 2016 12:10, Martin Koppenhoefer wrote:
> Why not (also) map the ones where it isn't allowed? You probably can't count on routing software implementing all country specific defaults (at least currently it isn't the case with the popular osm-based ones, so they need specific advice from the mappers in order to work well).
> To make sure that this is an relative exceptional situation in the context (country), for your fellow mappers? in any case you could also map the traffic sign, so your precious surveyed "insider knowledge" has better chances to persist in the db.
> From a practical point of view, routing engines will generally be very reluctant to suggest u-turns, because they tend to take a lot of time or might even be close to impossible (with lots of traffic). 
> This said, I could imagine restriction=allowed like you suggested. Logically, restriction=no would also make sense, but it bears the risk of being interpreted falsely (if someone checks for restriction=no* as short way for no_left_turn, no_right_turn etc.)
> cheers,
> Martin 
Authors of routing engines are, in fact, forced to make such a decision
for every traffic rule they intend to support. If they want to support
u-turns, they need to decide what to do if there are no explicit u-turn
rules available for each specific spot. Since the authors are unlikely
to research the default cases for all areas, I would guess they use pick
one to use for all countries and wait until someone files a bug report
stating otherwise for their area. With so many routing engines available
it's unrealistic for people in every country/area to check the defaults
to verify they conform with their area. This problem is not limited to
u-turns, of course.

It would seem to make sense, in these types of cases, to define the
general traffic rule of an area and focus on tagging the exceptions
within it. Doing otherwise would introduce unnecessary complexity to
their calculations and data bloat, or they would need to run a process
to strip out the traffic rules which conform with their default traffic
rule. This could be avoided if we provided the defaults either in the
data and/or via a wiki page the routing engine authors can check.

- Svavar Kjarrval

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