Traffic analysis zones (TAZs) are the basic geographic unit for inventorying demographic data and land use within a study area. While the total number of TAZs dictates the size of trip matrices, the size and shape of TAZs can influence model results. Most notably, highway and transit trip loadings and the percent of intra-zonal trips are directly impacted by study area zonal detail and the size of the zones. Consequently, defining an appropriate zonal geography has long been a challenge to the travel demand model community.
Historically, the number of zones in a study area was limited by the capacity and processing power of computers. With the advent of geographic information system (GIS) integrated software and increased micro-computer power and capacity, the maximum number of zones in a study area is much less restricted. Generally, the more zones a study area has, the more useful the model may be for various planning related purposes (e.g. transit modeling, different land use alternatives, studying non-motorized activities, sub-area analysis). Increasing the number of zones in a study area does require additional investment of time and resources. As one contributor noted, "the greater level of zone detail should be balanced with the extra costs associated with the development of detailed input data, model development and maintenance, increased file storage space requirements and increased model run times." Ultimately, the number of zones in a study area is determined by the study area's size and planning needs. The following is a brief synopsis of the contributions to the email list regarding general guidelines for defining TAZs.
Generally, the number of zones should not simply be a function of the study area population. There are several factors that contribute to the determination of the number of zones in a travel model, including network detail, potential future alternatives to the network and land use, data requirements of the model (e.g. auto vs. non-motorized travel), and the anticipated growth in the study area. Contributors to the email list offered the following suggestions for determining an appropriate number of zones:
Contributors also offered several observations regarding zonal structure:
Based on the guidelines and observations noted above, zones are typically defined to be compatible with:
Average zone sizes and ratios of zones per square mile provide useful insights about zone geography coarseness and may highlight potential trip assignment issues. Likewise, the average number of people per zone in a study area is a general indicator of regional density. A number of contributors noted additional metrics and statistics to assess network and land use compatibility with zone geographies. These included:
The density or size of zones can vary greatly depending on the amount of rural or less-developed areas in a study area. In addition, air quality requirements may require the study area to model outlying regions forcing the inclusion of more remote or developing areas, thereby influencing density ratios.
As one participant to the TAZ discussion noted, statistics regarding TAZ composition and size from a national survey sponsored by the Transportation Research Board (TRB) regarding travel demand model practices are available in the report entitled, "Determination of the State of the Practice in Metropolitan Area Travel Forecasting – Findings of the Surveys of Metropolitan Planning Organizations (Final Draft – June 2007)." The material is available at the following location: http://onlinepubs.trb.org/onlinepubs/reports/VHB-2007-Final.pdf on pages 22 and 23. The material is presented for small, medium and large size Metropolitan Planning Organizations.
Different zone structures for different steps of the model could be applied to make the number of zones more compatible with the particular travel model step. As one contributor added to the discussion, "transit and traffic assignments require more spatial detail than trip generation and trip distribution". For example, mode choice and traffic assignment models could have more zones than the prior trip generation or distribution steps, which may not need the same level of detail. Concerns regarding consistency and forecasting accuracy however, lead others to suggest that the best approach is to have the same number of zones for each model step.
A standard for determining if a study area has an adequate number of zones does not exist. While increased computer power has essentially eliminated restrictions on the number of zones for all but the largest study areas, increasing the number of zones does lead to increased data collection and maintenance costs. Greater zonal detail can improve transit and trip assignment results and extends the potential usefulness of the model for other planning applications and studies (e.g. migrating data to a micro-simulation package). Smaller zones, as some contributors noted, may also discourage the practice of simply implementing special centroid connector capacities or volume delay equations in order to improve the adjacent network loadings.
The objective of the series is to provide technical syntheses of current discussion topics generating significant interest on the TMIP e-mail list. Each synthesis is drawn from e-mails posted to the TMIP email list regarding a specific topic. The syntheses are intended to capture and organize worthwhile thoughts and discussions into one concise document. They do not represent the opinions of FHWA and do not constitute an endorsement, recommendation or specification by FHWA. These syntheses do not determine or advocate a policy decision/directive or make specific recommendations regarding future research initiatives. The syntheses are based solely on comments posted to the e-mail list.