Accessibility is a concept which either looks at how easy it is to reach a certain location from many other locations, or how easy it is to reach other locations or opportunities from a given starting point. This chapter looks at the latter, discussing that this is a quantity that can be defined separately for every point (x,y) in space, rather than treating accessibility as uniform within, say, zones. As a result, accessibility can also be seen as a continuous field A(x,y) in the two-dimensional environment. The chapter then continues to discuss how A(x,y) can be efficiently computed for regional scenarios. The approach combines interpolation of values computed on a grid with fast shortest-path tree computations and information caching for repeated sub-computations of the same quantities, using the econometric logsum term as an example of a possible indicator of accessibility. A Zurich scenario needs about two minutes of computing time on a regular desktop computer in order to compute A(x,y) at a resolution of 100 m x 100 m. As a sensitivity study, workplace accessibility maps are given for free speed car, congested car, bicycle and walking. One can for example observe that accessibility by bicycle is similar to congested car accessibility within the urban area, while it is worse outside and considerably worse when compared to free speed car transport. Similarly, walking accessibility is similar to bicycle and congested car transport in the innermost urban core, but considerably worse everywhere else.
