Clarify spec: Add definitions to Cause and Effect values and clarify Trip Update precedence for consumer routing decisions

[skalexch]

Summary

MobilityData is currently developing Service Alerts guidance to help the community use Service Alerts more consistently. As part of this work, we are gathering feedback through the Service Alerts Working Group meeting.

In the Apr 28th, 2026 meeting, we presented and discussed detailed definitions of values for the Cause and Effect fields, drafted here, to make sure that producers are attributing the correct values to real-world cases.

In addition, we discussed how consumers should interpret cases where both Trip Updates and Service Alerts apply but provide conflicting information. The group reached consensus that consumers should by default give precedence to Trip Updates for routing decisions.

Proposed Solution

This PR

1. Adds a clarification to feed-entities.md describing the expected consumer behaviour when Trip Updates and Service Alerts both apply.
   While this clarification will also be included in the Service Alerts guidance, we believe it is appropriate to include it directly in the specification as well, in order to make this shared expectation clearer.

2. Details the definitions for values of Cause and Effect, to make sure that service alerts are correctly categorized.

These changes are intended to clarify definitions and to clarify a principle for expected consumer behaviour, and they do not introduce a new field or functional change. Therefore, I propose that it can proceed to a vote without requiring implementation testing.

Type of change

GTFS Schedule

• Functional Change
• Non-Functional Change
• Documentation Maintenance

GTFS Realtime

• Specification Change
• Specification Change (Experimental Field)

Additional information

Here is the working group meeting notes

We also plan to add an additional Service Alerts meeting in July to continue discussing guidance around Service Alert usage. Registration page

[tzujenchanmbd]

@skalexch I just noticed there are some similar cause/effect contents in the https://github.com/google/transit/blob/master/gtfs-realtime/spec/en/service-alerts.md. Could you also check if there is need to adjust anything on that page?

[skalexch]

@tzujenchanmbd I added the definitions. Now they exist both in service-alerts.md and reference.md