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< h1 > < a class = "header" href = "#multi-modal-trips" id = "multi-modal-trips" > Multi-modal trips< / a > < / h1 >
< p > A single trip consists of a sequence of < code > TripLegs< / code > -- walking, operating a
vehicle (car or bike), and riding the bus. Depending whether a trip begins or
ends at a border or building, there are many combinations of these sequences.
This is a way to categorize them into three groups. I'm not sure it's the
simplest way to express all the state transitons.< / p >
< h2 > < a class = "header" href = "#walking-only-trips" id = "walking-only-trips" > Walking-only trips< / a > < / h2 >
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< p > < img src = "../mdbook-plantuml-img/bbf8881e880a261530dd998679b4f47108dfc748.svg" alt = "" / > < / p >
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< h2 > < a class = "header" href = "#trips-starting-from-a-border" id = "trips-starting-from-a-border" > Trips starting from a border< / a > < / h2 >
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< p > < img src = "../mdbook-plantuml-img/1cd9d730c8982c5ba59b825b427e712fab82e681.svg" alt = "" / > < / p >
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< h2 > < a class = "header" href = "#trips-starting-from-a-building" id = "trips-starting-from-a-building" > Trips starting from a building< / a > < / h2 >
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< p > < img src = "../mdbook-plantuml-img/92ec1d5cbfccd87df09b67ac1a2b26aebf81eee8.svg" alt = "" / > < / p >
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< h2 > < a class = "header" href = "#spawning-code-overview" id = "spawning-code-overview" > Spawning code overview< / a > < / h2 >
< p > As of November 2020, starting a traffic simulation works like this:< / p >
< ol >
< li > Something creates a < code > Scenario< / code > , which defines a bunch of people. Each person
has a schedule of trips, carrying them between < code > TripEndpoints< / code > via some
< code > TripMode< / code > , leaving at some < code > Time< / code > .< / li >
< li > When a scenario is instantiated, every trip for every person passes through
< code > TripSpec::maybe_new< / code > . This transforms the origin, destination, and mode into
a < code > TripSpec< / code > .< / li >
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< li > < code > TripSpec::to_plan< / code > further validates these and attempts to repair impossible
plans. Each < code > TripSpec< / code > is turned into a list of < code > TripLegs< / code > . The < code > TripSpec< / code > is
then passed to the discrete-event scheduler inside a < code > Command::StartTrip< / code > .
This way, < code > TripManager< / code > knows about all trips immediately, and at the right
time, each trip can be initiated.< / li >
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< li > Later, the scheduler gets one of these commands, and calls
< code > TripManager::start_trip< / code > with the < code > TripSpec< / code > . Each type of trip has its own
initialization logic to kick off the first leg of the trip.< / li >
< li > Most trips have multiple legs. When the first car, bike, or pedestrian
reaches their goal, < code > TripManager< / code > gets called with some sort of transition
function to initiate the next leg of the trip, or declare the trip finished.
These transition functions also record stats from that leg of the trip, like
total blocked time.< / li >
< / ol >
< p > What're the different < code > TripSpec< / code > cases, and what < code > TripLegs< / code > do they get turned
into?< / p >
< ul >
< li > < code > VehicleAppearing< / code > : Starts at a border. Drive, then maybe walk to the
destination building.< / li >
< li > < code > SpawningFailure< / code > : No trip legs; just create and immediately cancel the trip.< / li >
< li > < code > UsingParkedCar< / code > : Starts at a building. Walk (to the parked car), drive, then
maybe walk to the destination building (after parking again).< / li >
< li > < code > JustWalking< / code > : Just walk between two buildings/borders.< / li >
< li > < code > UsingBike< / code > : Starts at a building. Walk to the nearest bikeable lane, drive,
then maybe walk to the destination building. (Note that starting a bike from a
border uses < code > VehicleAppearing< / code > .)< / li >
< li > < code > UsingTransit< / code > : Walk, ride the bus, maybe walk again. No transfers yet; only
rides one bus between two stops.< / li >
< / ul >
< p > < code > TripManager< / code > has a whole bunch of transition functions:< / p >
< ul >
< li > < code > car_reached_parking_spot< / code > : drive -> walk, unless the destination building is
where we parked< / li >
< li > < code > ped_reached_parking_spot< / code > : walk -> drive< / li >
< li > < code > ped_ready_to_bike< / code > : walk -> bike< / li >
< li > < code > bike_reached_end< / code > : bike -> walk< / li >
< li > < code > ped_reached_building< / code > : walk -> done< / li >
< li > < code > ped_reached_bus_stop< / code > : walk -> wait or ride bus< / li >
< li > < code > ped_boarded_bus< / code > : waiting -> ride bus< / li >
< li > < code > person_left_bus< / code > : riding bus -> walk< / li >
< li > < code > ped_reached_border< / code > : walk -> done< / li >
< li > < code > transit_rider_reached_border< / code > : ride bus -> done< / li >
< li > < code > car_or_bike_reached_border< / code > : drive -> done< / li >
< / ul >
< p > There are at least a few use cases motivating the cleanup of all of this
structure:< / p >
< ul >
< li > Capping trips through congested areas. Sometimes this just changes the driving
route, but sometimes it needs to cancel trips, convert driving trips to
walking/transit, or delay the trip's start.< / li >
< li > Multiple public transit rides in a single trip, aka transferring< / li >
< li > Handling live map edits in the middle of a trip< / li >
< / ul >
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