3.3 Short Distance Impedance Generation

Once the all-streets network preparation is complete, it can be used to generate short distance impedances. These short distance impedances are typically used in two ways: they are input to the microzone-level data preparation in order to provide more accurate measures of accessibilities to employment, households, schools and transit, and they can also be used directly by the DaySim choice models, in conjunction with more typical TAZ-based impedances. There is a multistep process for generating the short distance impedances. First, a list of network node pairs is generated consisting of all node pairs that are within 3 miles (Euclidean, based on node coordinates) of each other is generated using a utility program. Network shortest path distances are then calculated for each of the node pairs in the list using a roadway network pathbuilding tool called DTALite. DTALite is used because it is very fast, although other pathbuilding tools may also be used. The result is a list of node pairs with the all streets network shortest path distances between them that is an input to the microzone buffering process that is described in subsequent sections.