PT routing

Walking + PT has been started in this repo. Just starting a list of improvements.

# UI

- [ ] Just make a nice UI to explore GTFS data. See routes on a map, see timetables per stop, etc. Look for inspiration. Old ideas in https://github.com/dabreegster/bus_spotting and https://github.com/a-b-street/abstreet/issues/372.

# A-to-B routing

- [ ] Configurable start time
- [ ] Display text route directions nicer. "Walk 5 minutes", "wait 4 minutes", plumb info about the route taken
- [x] Correctness -- only ever 1 stop per trip? Impossible
- [ ] Use route shapes for display (and see how much space they need)
- [ ] Get more precise about linking walking and transit steps

# Isochrone

- [x] Implement it

# Data model

- [ ] Decide requirements for days of the week, service exceptions, etc
- [ ] Think through serveless-friendly ways of getting GTFS data

# Performance

- [ ] Can an A*-like heuristic help for PT routing? Edit: yes, but it loses optimality
- [ ] Get base file size down, without GTFS. London is 370MB, but the PFB is 90MB
- [ ] Smaller GTFS data. London is around 300MB right now
- [ ] More compact NaiveTimes?

# Refactoring

- [ ] Figure out what frontend and backend components are generally shareable
- [ ] Specifically use in https://github.com/Hussein-Mahfouz/meet-me-halfway/

The issue is a broad roadmap for public transit routing improvements. Start by exploring the existing code for GTFS data handling and routing algorithms. Look at the linked repositories for UI inspiration and check the current implementation of isochrones and A-to-B routing. Determine which specific sub-task is most feasible and has clear acceptance criteria.

Written by the indexing model from the issue text.