Requirements for interpreting missing predictions in a trip

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Sean Barbeau

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Jul 21, 2015, 11:32:18 AM7/21/15
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Hi all,
Does SIRI have a set of requirements defining how consumers should interpret missing predictions for stops in a trip?

For example, if a SIRI producer has the following stops:

* 1
* 2
* 3
* 4

And the following predictions:
 
* 9:15am
* --- no data
* 9:30am
* 9:45am

Is the consumer allowed to interpolate a prediction for stop 2?  Or is it required to simply propagate the estimated deviation from stop 1 downstream to stop 2?  Or is the consumer required to not show any real-time information for stop 2?

I'm asking this because GTFS-realtime has such a requirement - if a prediction is not provided for a stop, and an explicit "unknown" value isn't provided for that same stop, then consumers are required to propagate predictions downstream - see https://developers.google.com/transit/gtfs-realtime/trip-updates#stop-time-updates.

Any info is appreciated, especially if there are hard requirements in the SIRI spec.

Thanks,
Sean

Christophe Duquesne

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Jul 21, 2015, 1:21:07 PM7/21/15
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Hi Sean

If I understand well, this GTFS-RT feature is only for updates (so you are supposed to have a former message carying some reference passing times).

SIRI has nothing explicit like this delay propagation described for GTFS-RT (the example they give is weird, but this can be useful to get smaller message allowing to delay a whole journey by delaying only the first delayed stop).
You could decide this in a SIRI local agreement (or profile), but is would be valid only for people using this specific profile (we didn't make this choice in France for example).

This is of course for Line centric services (i.e. Estimated Timetable).

For Stop centric services (Stop Monitoring) in SIRI, if the provider has some trouble with a specific passing time, instead of not giving any info, he is encouraged to provide the passing time he has and fill the ExpectedDeparturePredictionQuality.PredictionLevel field.

More generally (for all services) the provider can also, at journey level, use the PredictionInaccurate, ConfidenceLevel or vehicleStatus. If the provider doesn't provide any information there is no rule inside SIRI itself that tells the recipient what to do. But in most local agreements and profiles it is said that you are not allowed to create any information if you don't receive some (you can't communicate on a information you didn't receive).

Christophe
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