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diff --git a/gtfs-book/ch-18-working-with-trip-blocks.md b/gtfs-book/ch-18-working-with-trip-blocks.md new file mode 100644 index 0000000..d59b2c4 --- /dev/null +++ b/gtfs-book/ch-18-working-with-trip-blocks.md @@ -0,0 +1,109 @@ +## 18. Working With Trip Blocks** + +One of the more complex aspects of GTFS is how to properly use the +`block_id` field in `trips.txt`. The concept is simple, but +incorporating this information in a manner that is simple to understand +for a passenger can be more difficult. + +A single vehicle (such as a bus or train) completes multiple trips in a +single service day. For instance, once a bus completes its trip (Trip X) +from Location A to Location B, it then begins another trip (Trip Y) from +Location B to Location C. It then completes a final trip (Trip Z) from +Location C back to Location A. + +If a passenger boards in the middle of Trip X and is allowed to stay on +the bus until the end of the Trip Y, then this should be represented in +GTFS by giving Trips X, Y, Z the same `block_id` value. + +The following diagram shows a vehicle that performs two trips, +completing opposite directions of the same route. Often a single vehicle +will do many more than just two trips in a day. + + + +***Note:** When trips in a block are represented in stop_times.txt, the +final stop of a trip and the first stop of the subsequent trip must both +be included, even though they are typically at the same stop (and often +the same arrival & departure time). The final stop of the first trip is +drop-off only, while the first stop of the next trip is pick-up only.* + +In the following diagram, the vehicle completes three trips, each for +separate routes. As in the previous diagram, the vehicle will likely +complete more trips over a single day. + + + +These two diagrams show the difficulties that can arise when trying to +calculate trips across multiple blocks. The first diagram is repeated +below, this time with stops marked so trips between two locations can be +found. + + + +Consider the scenario where you want to retrieve all trip times from +Stop S2 to Stop S1. Since the vehicle gets to Stop S3 then turns around +and heads back to S1, the trip search returns two options: + +1. Board at S2 on the upper trip, travel via S3, then get to S1. +2. Board at S2 on the lower trip, travel directly to S1. + +The second option is a subset of the first option, which means it is a +far shorter trip (plus it avoids the passenger getting annoyed from +passing their starting point again). To determine which option to use, +you can use the trip duration. + +Now consider the other type of block formation, where a vehicle +completes subsequent trips from different routes. + + + +If a passenger wants to travel from Stop S2 to S4, you will not get the +situation where the vehicle travels past the same location twice. +However, it is also possible for a passenger to travel from Stop S2 to +Stop S6 without ever having to change vehicles. + +Referring back to the previous chapter about performing trip searches, +it is relatively straightforward to account for blocks in these +searches. Using the query to find trips between two stops as a +reference, the following changes need to be made: + +* Instead of joining both occurrences of `stop_times` to the + `trips` table, you now need two occurrences of `trips`. +* Each `stop_times` table occurrence is joined to a separate + `trips` table occurrence. +* The two occurrences of `trips` are joined on the `block_id` + value (if it is available). + +The following query shows how to find trips that start at stop `S2` +and finish at stop `S4`, departing after 1 PM on the day with service +ID `C1`. + +```sql +SELECT t1.*, t2.*, st1.*, st2.* + FROM trips t1, trips t2, stop_times st1, stop_times st2 + WHERE st1.trip_id = t1.trip_id + AND st2.trip_id = t2.trip_id + AND st1.stop_id = 'S2' + AND st2.stop_id = 'S4' + AND t1.service_id = 'C1' + AND ( + t1.trip_id = t2.trip_id + OR ( + LENGTH(t1.block_id) > 0 AND t1.block_id = t2.block_id + ) + ) + AND st1.departure_time >= '13:00:00' + AND st1.pickup_type = 0 + AND st2.drop_off_type = 0 + AND st1.departure_time < st2.arrival_time + ORDER BY st1.departure_time; +``` + +Since `block_id` may be unpopulated, the query joins on both the +`trip_id` and the `block_id`. + +***Note:** An alternative is to guarantee every single trip has a +`block_id` value when importing -- even if some blocks only consist of +a single trip. If you can guarantee this condition, then you can +simplify this query by using `t1.block_id = t2.block_id`.* + |