All Terms

acyclic subgraph

  »   An acyclic subgraph is a network without cycles (a cycle is a complete circuit). When following the network from node to node, a node is never visited twice.
adjacent notes

  »   Nodes that are reachable from a node via a single link
arrival time window

  »   A timeframe within which all paths that the passenger considers must arrive at the destination
backward shortest path

  »   A the shortest path computation that starts at the destination node. This is used when there is a preferred arrival time.
block

  »   A list of trips operated by the same vehicle. The second trip in the block cannot be operated before the first trip is completed (plus usually a layover).
common lines

  »   Concept that a traveler will board a vehicle with a longer expected travel time to the destination if the vehicle arrives first and the expected time conditional upon the arrival of the vehicle is less than the expected time of travel on an ostensibly faster route for which the passenger must wait to board (Chiriqui and Robillard, 1975).
departure time window

  »   A timeframe within which all paths that passenger considers must depart the origin.
destination

  »   A node that represents a person trip end-point.s
dispersion parameter

  »   Used for modeling stochasticity. This parameter is between 0 and 1. Higher values for the parameter mean that higher cost links within a hyperlink are devalued compared to the lowest cost links in the hyperlink. That is, if the lowest cost for a hyperlink is c, when a new link gets added to the hyperlink with a cost higher than c, when θ is closer to 1, the more the new link has a low probability compared to the lowest cost link. The higher θ emphasizes the lowest cost link more.
downstream link

  »   A link that is reachable from a node (forward shortest-path) or a link that can reach the current node (backward shortest path)
downstream node

  »   A node that is reachable from a node (forward shortest-path) or a node that can reach the current node (backward shortest path)
departure time window

  »   A timeframe within which all paths that passenger considers must depart the origin.
drive access link

  »   A link that connect a drive access point and an origin via non-transit vehicle. Generally exist for a continuous period of time.
drive access point

  »   A node that represents an access point to transit via driving such as a parking lot or drop-off point.
drive egress link

  »   A link that connect a drive access point and a destination via non-transit vehicle. Generally exist for a continuous period of time.
dynamic passenger assignment model

  »   A model that simulates the performance (reliability) and dynamics (capacity constraints, interactions) of a system of passenger demand and scheduled or on-demand transportation services (transit, TNCs, taxis) that run on a physical transportation network.
dynamic transit passenger assignment model

  »   A dynamic passenger assignment model that is designed for scheduled transit service.
emanating link

  »   Links coming into or going out of a node.
forward shortest path

  »   A shortest path computation that starts at the origin node. This is used when there is a preferred departure time.
hyperlink

  »   A hyperlink consists of a set of transit links from a single origin to multiple possible destination nodes [ for forward-shortest-path finding ] or a set of links from a single destination to multiple possible origin nodes [ for backward-shortest-path finding ]. There may be multiple transit departures within a hyperlink.
hyperpath cost

  »   The disutility of a specific hyperpath or portion thereof for a specific user class. Based on both link features, path features, and the path weight parameters.
hyperpath

  »   An acyclic subnetwork with at least one link connecting the origin to the destination, and where at each node, there are probabilities for choosing the alternative links. In most hyperpath-based frameworks, this can be equivalent to the path choice set.
joint person tour

  »   A person tour where one or more person trips is taken jointly with another person.
joint person trip

  »   A person trip that undertaken with another person.
kirchoff model

  »   An alternative formulation to the path-size logit model which considers the relative size of the total utilities of the path. For example, deltas of short paths are perceived differently than deltas on long paths. For paths that are generally the same size, the results will be similar to a logit model formulation.
departure time window

  »   The ordered queue that contains stops with stop states that need to have their costs updated in order to find the shortest path. Stops are processed in order of cost, with least cost stops processed first.
link feature

  »   A feature that can be accurately represented for an individual link (e.g. in-vehicle-time, wait-time, walk-time). Some features, like fares, can be represented at a link level, but may not be accurate if used without understanding of the entire path due to dependent features such as maximum fares or transfers.
link

  »   a connection between nodes.
node

  »   a specific point in space that represents a feature of the transit network
non-motorized access link

  »   A link that connects a transit stop and a destination via foot or other form of non-motorized travel. Generally exist for a continuous period of time.
non-motorized egress link

  »   A link that connects a transit stop and an origin via foot or other form of non-motorized travel. Generally exist for a continuous period of time.
observed passenger path links

  »   A list of links for observed paths, with their timing and costs, that a passenger has been observed taking. Often gained from an on-board survey or GPS data.
observed passenger path

  »   A path, with waypoints, timing and costs, that the passenger was observed taking. Often gained from an on-board survey or GPS data. Full observed paths with all applicable waypoints can be difficult to obtain and verify the validity of. For example, on-board surveys often use a combination of stated, revealed, and intended paths because passengers are often intercepted mid-trip. GPS data often has complete revealed data, but can be subject to GPS errors, which are most drastic in urban environments. Therefore, observed passenger paths often have a great deal of interpolation and assumptions built into them that are important to understand when using them to evaluate a model.
observed transit vehicle passenger activity

  »   The observed ridership activity observed via passenger data collection methods such as APC or similar.
observed transit vehicle trajectory

  »   Trajectory of a transit vehicle as observed from automatic vehicle location (AVL) data or similar.
origin

  »   A node that represents where a person trip starts.
overlap scale parameter

  »   The parameter in the path-size logit model that scales the impact of the path size term.
passenger assignment

  »   For all unassigned passenger-trips, select a path from the passenger-trip’s pathset based on probabilities calculated from a path-choice model and pathset costs and summarize vehicle loads.
passenger path links

  »   A list of links within passenger paths, with their timing and costs.
passenger path set generation

  »   The process of initializing the label stop queue, updating, and then finalizing the stop states. The origin or destination state now has a label that has a cost that encapsulates the costs of all the trip links and transfers, but with inaccuracies regarding the timing of the non-transit links, which must be updated using path enumeration.
passenger paths

  »   A list of paths, with their timing and costs.
person-trips

  »   A list of person trips with information required to assign them to vehicles.
passenger simulation

  »   Probabilistic assignment of passengers to paths in their pathset based on costs. Update of pathset feasibility.
path choice model

  »   A mathematical model for selecting a path from a set of feasible choices. Examples include the recursive logit model, path-size logit model, Kirchhoff model.
path cost

  »   Path cost or generalized cost is a measure of impedance on a network object. It is typically composed of different variables, each with a fixed weight. Terms may include cost, in-vehicle time, number of transfers, wait time etc.
path enumeration

  »   Walk the labeled hyperpath and generate all of the actual, realizable paths from it. At this point, we can fix the timing of the walk links and therefore have actual wait times. The path costs here are used to calculate the probability of each of these paths. The output of this process is a pathset.
path feasibility check

  »   Flag passenger trips that are not on a valid path due to missed transfers or overcapacity vehicles based on capacity priority rules.
path feature

  »   Feature of a path that is summarized for the entire path. Can be an aggregation of the link features comprising the path (i.e. wait-time), or a variable that can only be calculated at the level of the entire path (e.g. fare, directness).
path labeling

  »   The process of initializing the label stop queue, updating, and then finalizing the stop states. The origin or destination state now has a label that has a cost that encapsulates the costs of all the trip links and transfers, but with inaccuracies regarding the timing of the non-transit links, which must be updated using path enumeration.
path overlap variable

  »   An additive measure of similarity between paths. In road-based path-choice models, this is often the distance of the shared links. In passenger-based path-choice models, this could include considerations for shared-routes, similar-routes, on/off stations, traversed stations, traverse distance and more. If the overlap variable is an indicator variable (𝛿), then it can be 1 or 0; if it distance or cost, then it is a continuous variable.
path overlap

  »   In a set of possible paths through a transportation network, some portion of each of the paths may share a facility meaning that each choice in a choice set is not mutually exclusive. This is important in the context of choice modeling, since it violates the “Independence” in the IIA property of a Multinomial Logit. Formulations that compensate for this violation by discounting the “independence” of each path based on a measurement of commonality (the path overlap variable) include the path-size logit model.
path size logit model

  »   A modified logit choice model where the utility equation for a path adds a path size variable to the utility of a path alternative in order to account for overlap between different path options.
path weight parameters

  »   Coefficients on various components of the utility equation for the path choice model. Can be estimated, asserted, or inferred. Path weight parameters can either be based on a link feature (i.e. in-vehicle time) or path feature (i.e. directness). The path weight parameters can vary by user class.
pathfinding cost

  »   The cost of each path as calculated during pathfinding, typically including costs of each leg plus some non-additive costs. Because of the non-additive components (e.g. fares) and the adjustment of some components (e.g. travelers may leave their origin later than their preferred time to match their bus departure time), it’s typically computed fully only after the path is fully defined (origin to destination and all legs in between). Differs from the simulation cost in that the travel times may not be accurate because the simulation step hasn’t happened yet which would load passengers onto vehicles, thereby slowing them down for boarding/alighting and possibly bumping passengers.
pathfinding probability

  »   The probability of each path in the pathset based on the pathfinding cost. Differs from simulated probability in that the simulation step hasn’t occurred yet so the probabilities are subject to change; for example, some paths may become infeasible if the simulation fills up some vehicles so the the passenger cannot board the vehicle for this leg. May also include path overlap penalties (although Fast-Trips doesn’t calculate it in Pathfinding, only in Simulation).
pathset update

  »   Update pathset paths based on transit vehicle trajectory cost updates and path feasibility.
joint person tour

  »   A set of paths between an origin and destination with specific costs, waypoints, and timings. In hyperpath-based frameworks this is often derived from the hyperpath.
person tour

  »   a series of chained person trips that emanate from a base location such as home or workplace.
person trip

  »   Travel from an origin to a destination by a single person.
predicted passenger path choice set links

  »   A list of links for a predicted path set/hyperpath, with their timing and costs, that the passenger considers taking.
predicted passenger path choice set

  »   A list of feasible paths, with their timing and costs, that the passenger considers taking for a specific trip. In some frameworks, hyperpaths can be used to create the path choice set.
predicted passenger path links

  »   The list of links for the path a passenger is predicted to take for a specific trip, with their timing and costs.
Predicted Passenger Path

  »   A path, with timing and costs, that the passenger is predicted take for a specific trip.
predicted transit vehicle passenger activity

  »   The ridership activity as predicted by a model.
predicted transit vehicle trajectory

  »   Trajectory of a transit vehicle as predicted from a model.
preferred arrival time

  »   The target time for the passenger to arrive. Is within the arrival time window. In trips with more rigidity surrounding the arrival time (i.e. commutes to work), the preferred arrival time is often used for finding a backwards shortest path to the origin.
preferred departure time

  »   The target time for the passenger to depart. Is within the departure time window. In trips with more rigidity surrounding the departure time (i.e. commuting home from work), the preferred departure time is often used for finding a forward shortest path to the destination.
schedule-based transit network

  »   A set of transit vehicle trips serving transit stops using a specified schedule.
scheduled transit vehicle trajectory

  »   The intended trajectory of a transit vehicle. Also known as transit schedule.
joint person trip

  »   A person trip that undertaken with another person.
simulated cost

  »   The cost of each path as calculated during simulation. It differs from the pathfinding cost in that it includes the effect of simulation (loaded vehicles and updated vehicle travel times, crowding costs or costs associated from being bumped from the vehicle).
simulated probability

  »   The probability of each path as calculated during simulation. Like the simulated cost, it differs from the pathfinding probability in that it includes the effect of simulation (loaded vehicles and updated vehicle travel times, crowding costs or costs associated from being bumped from the vehicle, which may make the path infeasible, giving it 0% probability).
stop label

  »   In the context of a shortest path algorithm, stops are labeled with the overall generalized cost of travelling from that stop to the destination (in a forward shortest-path) or from that stop to the origin (in a backwards shortest-path). Stops can be iteratively updated throughout the algorithm.
stop state finalization

  »   When all stops are removed from the label stop queue, the final costs for the destination (in forward-shortest-path) or origin (in backward-shortest-path) is finalized based on the cost labels of the emanating egress or access links.
stop state initialization

  »   The first step of a shortest-path algorithm. Stop labels should be initialized to be greater-than or equal to their final cost and should allow for the greatest number of emanating links (i.e. for a walk access link in a forward-shortest-path, assume it is as early as possible). All stops are added to the label stop queue.
stop state updating

  »   Updating stop states when a stop is removed from the label stop queue.
stop state

  »   The combination of a stop label (l) and upstream (forward shortest-path) or downstream (backward shortest-path) links (Pa).
stop

  »   A node that represents a transit stop.
transfer link

  »   A link that connects two transit stops or a transit stop and another mode (i.e. a drive access point). Generally exists for a continuous period of time.
transit in-vehicle time

  »   The time spent by the passenger on an actual transit vehicle.
transit link

  »   a link that connects two transit stops via a transit service. Generally exists for a specific, discrete time.
Transit Route

  »   A grouped pattern of transit service. For example, the 24-Divisadero.
transit schedule

  »   The intended trajectory of a transit vehicle. Also known as scheduled transit vehicle trajectory.
transit vehicle crush capacity

  »   Maximum number of people that can physically get on a transit vehicle.
transit vehicle design capacity

  »   Maximum number of people that a transit vehicle is designed to carry safely.
transit vehicle dwell time

  »   The time between when a vehicle opens and closes its doors at a bus stop (or is available to open/close them in the event the driver is conserving energy because of weather). This is the time window that a passenger can board or exit the vehicle.
transit vehicle passenger activity

  »   The ridership activity of a set of transit vehicles. Ons, Offs, Volumes by vehicle and location.
transit vehicle seated capacity

  »   Number of seats on a transit vehicle including seats that can be raised and lowered.
transit vehicle stopping time

  »   The time lost by a transit vehicle based on having to service a stop. This includes deceleration, dwell time, accelerating and merging (if applicable).
transit vehicle trajectory update

  »   If the results of the passenger assignment have an effect on transit vehicle timings (i.e. boarding and alighting activity at stop that affect vehicle dwell times), update the transit vehicle trajectories to reflect it.
transit vehicle trajectory

  »   The trajectory of transit vehicle is a list of stops that are visited at specified time as they are carrying out one or more transit vehicle trips. Could be based on transit schedule, but also could be updated based on passenger activities or other disturbances.
transit vehicle trip direction

  »   Which way a specific trip on a route is going. For example, inbound, outbound, northbound, southbound, clockwise, etc. Should be consistent throughout a system.
transit vehicle trip

  »   A single departure of a transit route going a specific direction. For example, the 24-Divisadero (inbound) that departs at 8:24 AM. This is expressed in GTFS as the trip_id.
transit vehicle

  »   The item that conveys passengers along a Transit Route. There is one transit vehicle per transit vehicle trip. A single transit vehicle can be compound in nature (i.e. a five-car train). Each vehicle has a capacity, and method of propulsion, acceleration, top-speed, and deceleration capabilities.
unassigned passenger pathset generation loop

  »   Passengers with no valid paths in the simulation loop, see if any new paths can be generated based on updated costs and available paths.
upstream link

  »   A link that is reachable from a node (backward shortest-path) or a link that can reach the current node (forward shortest path)
upstream node

  »   A node that is reachable from a node (backward shortest-path) or a node that can reach the current node (forward shortest path)
joint person trip

  »   A person trip that undertaken with another person.
user classes

  »   Passenger market segments who use the same parameters (including path weight parameters) to find and assess paths. Common user class segmentations are by travel purpose and captive versus choice users.
value of time

  »   The tradeoff between time and money, usually in units of dollars per hour.