Appendix F Glossary

The following is a list of some commonly used terms in transport modelling and their definitions.

Accessibility An indication of the proximity of a person, site or zone to a particular activity or group of activities. It is also defined as the ease or difficulty of making trips to or from each zone.
Aggregate data Data that relates to a mass of group of people, vehicles or area. The collective properties of the variable are of interest.
AON assignment The AON (all-or-nothing) assignment technique by which minimum travel time paths are computed for each zone pair and all flows between these pairs are loaded onto these paths.
Capacity restraint A traffic assignment technique that takes into account the build up of congestion with increased traffic volumes. It adjusts the link travel times according to the prevailing flows.
Centroid connector Imaginary links that represent the street network within a zone. They ‘connect’ trips from a zone to the modelled network.
Destination The point or area of termination of a trip.
Disaggregate data Data at the level of individual persons, households, etc.
Employment The number of employees, or jobs, in relation to the zone of work. This may be stratified by employment type e.g. retail, manufacturing, etc.
External trip A trip that has either an origin or destination, but not both, in the study area.
Equilibrium assignment An assignment process by which all used routes between zone pairs have equal and minimum costs, while all unused routes have greater or equal costs.
Generalised cost This cost is usually a linear additive function of some, or all, of the following costs: travel time between zones, access and wait times, ride time, distance between zones, fares, fuel costs and parking charges.
Gravity model A model that distributes the number of trips between all trip-producing zones and trip-attracting zones.
Home A group of rooms or a single room, occupied or intended for occupancy as separate living quarters by a family, group of persons or by a person living alone.
Home-based trip A trip that has its origin or destination at the home end. It may be a person trip, vehicle trip, walk trip, or public transport trip.
Household A person or persons living in the one home.
Incremental assignment The process by which flows between all zone pairs are loaded onto the network in pre-specified steps.
Internal trip A trip that has both its origin and destination in the study area.
Link A section of a highway or public transport network defined by a node at each end.
Logit model Also known as the ‘multinomial logit model’, it calculates the proportion of trips that will select a specific mode or activity.
Minimum path The route between a zone pair that has the least cost (time, distance, generalised) in comparison to all other possible routes.
Minimum path tree All the minimum paths between zone pairs that emanate from an origin zone.
Modal split The division of trips between different modes of travel (private transport, public transport).
Node A numbered point on a network representing a centroid or a junction of two or more links.
Non home-based trip A trip that has neither origin nor destination at the home end. It may be a person trip, vehicle trip, walk trip, bicycle trip or public transport trip.
Origin The point or zone at which a trip begins.
Person trip Any trip made by a person.
Screenline An imaginary line, usually along physical barriers such as rivers, railway lines or roads. Screenlines split the study area into a number of parts. Traffic classification counts, and possibly interviews, may be conducted along these lines to compare or calibrate data and models.
Travel time The time taken to travel between two points.
Trip A one-way movement from an origin to a destination for a particular purpose. It may be a person trip, a vehicle trip, walking trip or public transport trip.
Trip assignment The process by which flows between zones derived from the trip distribution process are allocated to the minimum path routes through a network.
Trip attraction Usually used to describe trip ends connected with non-residential land uses in a zone. Also defined as the non home end of a home based trip or the destination of a non home based trip.
Trip distribution The process by which the total numbers of trips originating in each zone are distributed among all the possible destination zones.
Trip end Either a trip origin or trip destination.
Trip generation The process by which the total numbers of trips beginning, or ending, in a zone are determined, based on demographic, socio-economic and land use characteristics.
Trip matrix A two dimensional matrix that represents the demand for travel among all zones in a study area for individual or grouped purposes, modes or types.
Trip production Usually used to describe trip ends connected with residential land uses in a zone. Also defined as the home end of a home-based trip, or the origin of a non home-based trip.
Trip purpose This can be defined as work trips, school trips, recreational or social trips and shopping trips.
Zone A portion of the study area with homogenous land use, socio-economic and demographic characteristics.
Zone centroid An assumed point in a zone that represents the origin or destination of all trips to or from that zone. Generally, it is the weighted centre of trip ends, rather than the geometrical centre of a zone.