Transportation Impact Analysis

Significance and Process Components

Better decisions can be made when impacts are accurately projected through the traffic analysis process. Mindful planning results in less future traffic congestion, fewer safety issues, milder environmental impact, and avoidance of unnecessary and costly improvements.

The major elements of a TIA include existing conditions analysis, estimation of future traffic with and without development, analysis of traffic impacts, and recommended roadway improvements. A number of variables are involved in, and data collected for, the assessment, [Page 1517]depending on the type, size, and location of development proposed. These typically include trip generation rates, trip distribution, peak morning and evening traffic flows, intersection capacity, vehicle demand, turning movements, and mitigation.

Some impact studies include data on transit, pedestrian, and bicycle traffic, as well as on parking, freight loading, and construction activities. Depending on the municipality and state in which the development is proposed, specific process guidelines are available for developers and transportation authorities.

Examples and Models

The study area of a site is determined and the project setting and description are compiled. These include existing and proposed total gross square footage for each land use type, description of existing transportation infrastructure, and description of change in function of lanes, public rights-of-way, and access sites. The initial overview should identify study issues, needs, assumptions, procedures, available sources of data, past and related studies, report requirements, and other topics relevant to prepare the analysis.

Other considerations during the overview phase of any development project are horizon year of buildout related to the proposal; other developments currently approved or under way; number, locations, capacity, and safety issues related to access points; and potential mitigation opportunities.

After the initial information is compiled, a travel demand analysis and traffic impact analysis need to be performed. Data collection and interpretation will usually include trip generation and distribution rates. Trip generation (simply put: in-and-out traffic) is the estimate of daily person-trips for an average weekday generated by households within each study area.

The Trip Generation Manual published by the Institute of Transportation Engineers (ITE) is often used as a guide and handbook to calculate these rates. Mainly, estimates are determined by data on weekday morning peak-hour and weekday evening peak-hour trips in residential areas.

There are typically two types of use for trip generation rates: (1) a transportation planning process that involves large zones of urban areas, with a goal to forecast the number of trips that will begin from or end in each travel analysis zone (TAZ); and (2) planning or forecasting based on trips made to a particular land use site, with a goal to determine impact of development, or land use changes.

...