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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 exactly was the Dutch national network model built, with which data?	Network model documentation (PBL)
Which GeoDMS operator or function do I use, and what is its syntax?	GeoDMS wiki

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 and published indicators
• PC4 travel time matrix for Nederland van Boven (in Dutch) — downloadable dataset

Reference

• Glossary
• References