Traffic Forecasts

Traffic forecasts are estimates of the number of vehicles or people taking a given route or using a given road (or other transportation project, from a specific bridge, to the 3:10 p.m. bus, to the Delta flight from Cleveland to Austin) in a given period of future time, and of the conditions of traffic as a result of that number and other available data. Such forecasts inform transportation planning decisions, and multiple forecasts can be used to choose between various options.

Traffic forecasts are a necessary part of the most basic parts of transportation planning; the decision whether to construct a commuter rail line from a city to a suburb depends on an accurate forecast of how [Page 1366]many residents of that suburb will use it to commute to the city, for instance, and the mapping of school bus routes depends on knowing how many students live along a given proposed route.

Traffic forecasts also impact decisions at higher levels of policy, such as the setting of speed limits, the deployment of highway patrol vehicles, building an expansion of an airport, or picking a location for a new school, park, or playground. They are also necessary components of other analyses, such as environmental impact statements, social impact statements, and the cost-benefit analyses of many transportation and infrastructure projects.

Traffic forecasts may also be prepared for a specific type of scenario rather than a specific period of time, such as forecasting the impact of a road closure, construction project, or weather event. Weather event traffic forecasts are important in developing weather emergency plans for a city or region, because the impact of flooding, fires, hurricanes, or ice and snow on traffic affects evacuation plans and the dispatch and deployment of emergency personnel, as well as other basic city functions.

The geographical area considered by a given traffic forecast or other transportation planning model is called a traffic analysis zone (TAZ). Zones may be of any size, but modern urban planning software usually assumes a zone with a population of less than 3,000. Census information provides population and socioeconomic data, and the specific types of data included in analyses vary from institution to institution, with each state's own Department of Transportation (DOT) typically providing a guidebook detailing which data to include. Usually, the number of automobiles and household income are both included, as they are valuable indicators of transportation usage and mode choice.