Transit Alerts are a way to quickly push information to applications like Google Maps and the Transit App. While usually used for emergencies, such as weather-related service disruptions, Transit Alerts can and should be used for some changes that may be able to be made in GTFS, but due to scale or timing, may not be the most efficient use of time to describe using GTFS.
What do Transit Alerts do?
Transit Alerts can tell consuming applications, like Apple Maps, that a stop is unavailable for trip itineraries, or a route is out of service, or service is being affected by weather or construction. There are several types of alerts which have different effects on how the applications will return search results to riders.
How do I know whether to use a Transit Alert or update the GTFS?
Generally speaking, GTFS is used to describe scheduled or expected service levels. When these change, such as a new permanent routing is employed or the scheduled timetable is updated, GTFS should be used to describe them. When changes are short-term though, or are not meant to be permanent (until the next schedule change, that is), it may be better to use an alert.
Good Transit Alerts
Stop Closure: This one can be very time consuming to describe in the GTFS, especially when a stop is used on many routes or patterns. While stops that are permanently closed should be removed from the GTFS, stop closures due to construction can be easily marked as closed using a transit alert without needing to update the GTFS.
Headway or service level disruptions: Some kinds of schedule changes, even when known, can be quite time consuming to generate. A good example is when service level will drop from 15 minutes to 20 minutes because of a driver shortage. If the current schedule is expected to stay the same, and the only change is the ability to meet that service level because of a driver shortage (especially a temporary one), describing service reductions using Transit Alerts can be a very efficient way to notify riders that there may be delays in service.
Early service end: Sometimes service may end prematurely, such as in anticipation of an event or a holiday. Using a systemwide Transit Alert to end service at 6pm on Christmas Eve reduces the need to maintain a separate exceptions calendar. If holidays are known and the service reduction remains the same across holidays, it is usually a good idea to put that into the GTFS so riders planning ahead of time see expected service levels, but if the reduction remains unknown until much closer to the event, such as a baseball playoff game, it may be easier and faster to describe this with an alert.
Good GTFS updates
Detours affecting CAD/AVL and realtime predictions: If you have a construction detour that is significant and your CAD/AVL system is using GTFS, it may be necessary to update the GTFS in order to ensure rider predictions or OTP remains strong. If your CAD/AVL system is disconnected from your GTFS, you may be able to make adjustments in the CAD/AVL system and use a Transit Alert for your static GTFS.
Low-frequency/headway routes with significant reductions: If you have a route that only runs every hour, it may not do a rider any good to know that there is now a reduction that could stretch that headway to as much as 2 hours. When using Transit Alerts to describe a service reduction, it works best if the headway remains frequent enough that a rider will not be stranded for a long period of time while they wait for a bus. In cases where headways are more than 20 minutes, using GTFS to update the schedule (especially if the reduction is in place for more than two weeks) may give your riders the best information possible.
Replacement services: If a route, or series of routes, will be replaced by a different service (such as a bus bridge in lieu of light rail service,) riders will be better serviced by GTFS updates. This ensures riders with mobility or access concerns can have them addressed before beginning their journey, as some forms of the alternate transit may not meet their needs.