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Accessibility Modelling

Accessibility is the ease with which relevant destinations can be reached from a given location. It is a driving factor in land use modelling, a policy indicator in its own right, and an input to a wide range of social, economic and environmental analyses.

This wiki is a method reference. It describes how accessibility is modelled: the concepts, the modelling choices, the indicators and the pitfalls, independently of any single project.

What you will find here, and what you will not

Question	Where to look
Which accessibility indicator fits my research question, and what do I need to compute it?	This wiki
Which projects has Object Vision done on accessibility?	objectvision.nl, accessibility
How was the Dutch national network model built, with which data and for which modes?	Network model PBL
How is the worldwide network built from OSM, and what accessibility products does it produce?	NetworkModel World
How is the European network used for school and service accessibility and location optimisation?	NetworkModel EU
Which GeoDMS operator or function do I use, and what is its syntax?	Network functions and impedance options on geodms.nl

Examples in this wiki use the GeoDMS where a concrete implementation helps, but the methods themselves are software independent.

Contents

1. Concepts

• Accessibility, what accessibility is, its four components and four measurement perspectives
• Travel impedance, cost, time and distance as the input to every accessibility measure
• Euclidean versus network distance, why "as the crow flies" is rarely good enough

2. Networks

• Network data sources, choosing between OSM, NWB, GTFS and other sources
  • OSM · NWB · GTFS
• Building a routable network, from geometry to a connected, directed graph
• Network optimisation algorithm, reducing a network without changing its routing behaviour
• Travel speeds per mode, car, bicycle, pedestrian and public transport

3. Origins and destinations

• Zoning and spatial units, points, zones, grids, and the errors each introduces
• Connecting origins and destinations to the network, connectors, snapping and access time

4. Computation

• Shortest path and the Dijkstra algorithm, the routing core
• Origin Destination matrix (trip table), many origins to many destinations
• Isochrones and catchment areas, reachable areas and service areas
• Public transport routing, timetables as time expanded graphs

5. Indicators

• Accessibility indicators, the indicator families side by side, and when to use which
• Distance decay functions, translating impedance into travel potential
• Spatial Interaction Models, the theory behind gravity and doubly constrained models
• Network centrality, centrality from random walker distributions

6. Applications

• Applications, projects, published indicators and downloadable datasets

Reference

• Glossary
• References