Description: Traffic analysis zones (TAZs) are universally used in travel demand modelling to represent thespatial distribution of trip1 origins and destinations, as well as the population, employment andother spatial attributes that generate or otherwise influence travel demand. The urban area isdivided into a set of mutually exclusive and collectively exhaustive zones. While travel actuallyoccurs from one point in the urban region to another, all trip origins and destinations in a traveldemand model are represented at the spatially aggregate level of the movement from an originzone to a destination zone. These movements are further aggregated within network assignmentmodels as originating and ending at single points within the origin and destination zones – thezone centroids.The road and transit networks coded into the computer network model reflect this zonalaggregation of space in that they are designed to carry the flows between zones, not withinzones. Zone centroids (which are the “sources” and “sinks” of trips) are connected to the roadand transit networks via artificial links known as centroid connectors which are a highlyabstracted representation of the actual fine-grained local street network, thus representinganother element in the spatial aggregation associated with the use of traffic zones.2The choice of a TAZ system is thus a first, very critical step in travel demand modelling, since itrepresents the fundamental level of spatial representation, precision and accuracy in the model.Despite this importance, TAZ definition has received very little attention in either the academicor professional literature. A review of the recent academic literature, frankly, provides littlepractical guidance for operational traffic zone design; nevertheless, several of the citations foundare included in the list of references. Similarly, a scan of the Transportation Research Board(TRB), Travel Model Improvement Program (TMIP) and Travel Forecasting Resource(TFResource) websites turns up little in the way of useful guidance. The one excellent exceptionfound to this statement is Cambridge Systematics (2007), which provides excellent, detailedguidance concerning TAZ design (even if it is US-oriented, as well as sometimes Floridaspecific).3 This report is recommended as “required reading” for any agency consideringreassessment of its traffic zone system, and it represents the starting point for the discussionpresented below. In particular, this report does not attempt to reproduce/summarize theCambridge Systematics report in any way, but rather augment it with what is hoped to be keypoints concerning TAZ design from an explicit travel demand modelling perspective.