Chapter 13 - Traffic Forecasting and Analysis Concepts

13.4. Trip Generation and Assignment for Traffic Impact Studies

13.4.2. Traffic Assignment

Traffic assignment is the process of placing site-generated trips onto the roadway network within the study area. Traffic assignment is done either manually or with modeling software. Traffic assignment for small to medium sized developments is more commonly handled with manual methods, while modeling software is often used for larger developments that have a regional impact. The site-generated trips (usually vehicles per peak hour) are added to the “background” traffic, which usually consists of the existing peak hour turning movement volumes at each intersection plus additional turning movements which account for compounded annual growth and sometimes traffic attributed to other nearby developments. The combined site-generated and background traffic form the total assigned traffic (intersection turning movements) that is used to measure level of service and determine necessary roadway improvements to accommodate the new development.

Traffic Assignment for Phased Developments

Many large developments are constructed in several phases over a period of years. The traffic impact study can reflect this reality by analyzing one or more intermediate phases, plus the full build-out scenario. Each new phase will assign additional traffic onto the assumed roadway network for that year. Background traffic for each new phase must include traffic assigned from previously opened phases of development.

Traffic Assignment of Three Major Trip Types

The three major trip types are primary trips, passer-by trips, and diverted trips. Each trip type will be separated when assigning site-generated traffic throughout the study network. This makes it easier for the reviewer to follow the assignment process and identify errors.

Primary trips are made for the specific purpose of visiting the development and they are new trips on the roadway network. Traffic will be assigned for primary trips throughout the study network according to the trip distribution percentages to and from the study area.

Passer-by trips are trips made as intermediate stops on the way from an origin to a primary destination. Passer-by trips are attracted from traffic already on adjacent roadways to the site. These trips are separately assigned to the study network only at site-access intersections and on internal circulation roadways within the site development itself. Turning movement volumes will be added at these intersections for entering and exiting traffic, while the through movements will be reduced by an equal amount. 

Diverted trips are similar to passer-by trips except they are attracted to a development from a nearby street or roadway that is not directly adjacent to the site development. Like passer-by trips, diverted trips are not new to the roadway system overall, but their route will include off-site roadways and intersections on the study network. Like passer-by trips, these volumes will be deducted from the through traffic on the original roadway that they were traveling on, and the diverted volumes will be added to the revised route to and from the new developments. For more information on passer-by and diverted trips, please refer to the ITE Trip Generation Handbook, a companion to the ITE Trip Generation. The Handbook also includes helpful insight in preparing traffic impact studies, including studies for multi-use developments.