The practical use is that routing algorithms can calculate the travel time more accurate if the durations are known. By specifying the via ways a routing algorithm knows which signals can be ignored. Simulations can be done more precisely.
To reply to the other comments: I am aware of the problem of dynamic timings. Nevertheless it is important to have a rough idea how these traffic signals are timed. The suggestions of Nakaner to tag the timings with fractions like green=40/120, indicating that the signal is 40 of 120 seconds green seems like a good compromise between complexity and loss of information. To increase the information a use of conditional fraction timings for different times at a day also seems to be a reasonable optional tool. I think the connection between more intersection is to complex to model and can be guessed of the simulation and routing applications. To know the exact via ways is a crucial feature for routing and simulation tasks. Relations seem to be a good way to model them. Also it seems to be more easy to tag on all relations which include certain nodes than to tag the information in the single nodes. Why do you think that it is better not to use relations? I’d like you to use the discussion page for a better readability http://wiki.openstreetmap.org/wiki/Talk:Proposed_features/Traffic_Signals <http://wiki.openstreetmap.org/wiki/Talk:Proposed_features/Traffic_Signals> > I'm going to have to agree with this. What's the practical purpose of this > schema? I the only thing I can come up with would be to recreate the > system for a driving game of some realistic fashion, however, that seems to > be more of something the game designer would need to implement on their own > rather than expecting some overly baroque tagging model in a third-party > database (ie, OSM) to sort out for them. The GPS navigation model doesn't > really bring up a solid case for this, either (with the exception of turn > lanes that prohibit turn on red and have their own signals, in which > there's bound to be a simpler way to indicate the presence of left or right > turn signals and whether or not they allow turn on red to give navigation > systems finer grain control over cost assignment, ie, in the pacific > northwest US model, a right turn would have the same penalty as a two-way > stop, as would a left turn to a one-way street (since you can make a left > on red arrow if it's to a one-way street in that part of the world, even > from a two way), but all other left turns involving a left turn signal > would "cost" more). > > I guess we're going to need a better idea of what this is trying to solve > beyond just mapping for the sake of detail overkill (which I have nothing > against, but let's not make things overly complex just for the sake of > this).
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