Add that headsigns are recommended

[isabelle-dr]

This PR closes #480.

Context and issue

Headsigns is one of the top-used optional field in GTFS. 82% of the datasets form the Mobility Database have it, which is more than route colors or feed_info.txt.
It's a field that often can and should be included and we believe there is value in calling it out specifically using the Best Practices (Recommended or Should), to promote higher-quality GTFS.

Discussions in the GH issue surfaced certain cases where there are no headsigns:

1. Loop routes
2. Flex datasets

Solution proposed

Adding a statement that headsigns is recommended for all services with headsign text displayed on the vehicle which may be used to distinguish amongst trips in a route.

I acknowledge this isn't something we can easily validate automatically and trigger a WARNING for it in the Canonical GTFS Schedule Validator.
That being said, the spec contains instances of must/required and recommended/should that can't easily be checked by an automatic validator and would have to be checked by other means, and we think this is definitely better than nothing. For example:

• In stop_times.stop_id: All stops serviced during a trip must have a record in stop_times.txt.
• in pathways.min_width: This field is recommended if the minimum width is less than 1 meter.
• In agency.agency_email: This email address should be a direct contact point where transit riders can reach a customer service representative at the agency.

Recommendations don't invalidate a GTFS dataset, this change is considered backwards-compatible.

Alternatives considered

In the GitHub issue, we've discussed adding a new trips.has_headsigns field that would have the Recommended status and make trips.trip_headsign Conditionally Required if has_headsigns==1, but then concluded the simple statement would lead to a good enough outcome with less complexity added to the spec.

Tagging folks involved in the GH issue @westontrillium @skinkie @antrim @evansiroky

[evansiroky]

Sorry for not commenting on the issue, but I'm voting +0 on this. I would have preferred to have some language in the spec to enable a way to code the data in a way that explicitly says there are no headsigns on the trips.

[isabelle-dr]

@evansiroky how about opening a separate GitHub issue for adding the functionality of indicating explicitly if a service has headings or not?
This PR won’t conflict with adding that feature later on.

[isabelle-dr]

I imagine the thumbs-up from @westontrillium @antrim @derhuerst means a general OK.
@evansiroky, I understand that you'd also want the spec to be able to explicitly indicate whether a service has headsigns 'in real life.' However, I believe this could be addressed in a separate GitHub issue and does not affect the relevance of the changes in this PR.

This PR has been open for at least 7 days, as per the Spec Amendment Process, I am opening a vote to make headsigns a recommended field in GTFS.
Voting ends on 2024-08-15 at 23:59:59 UTC.

[gcamp]

+1 Transit

[westontrillium]

+1 Trillium

[kylerchin]

+1 Catenary Maps. Some agencies don't provide a head sign and it's confusing.

[isabelle-dr]

The voting period ended on 2024-08-15 at 23:59:59 UTC.

With 3 votes in favor and no votes against, the vote passes.
The votes came from:
@gcamp (Transit App)
@westontrillium (Trillium)
@kylerchin (Catenary Maps) - 👋 welcome!

Thank you to everyone who participated!