The Subtleties of Trip Generation

September 30, 2016 by Dan Hennessey

This is the third in a three-part series regarding rough proportionality and its effect on development projects.  Here are links to Part 1  and to Part 2.

Part 1 of this series introduced rough proportionality and discussed why impact fees are used, and how they can be beneficial to both jurisdictions and, in some cases, developers.  Part 2 discussed how impact fees are determined, what is included in impact fee calculations, and for what they can be used.  This post will discuss important variables that can affect vehicle trip generation (and thus impact fee/rough proportionality payments).

The most important chapter of a transportation impact analysis is rarely the “Plus Project” section in which impacts are determined and mitigations stated.  That section merely shows the results of decisions made in an earlier chapter, which discusses the method used to estimate the amount of traffic generated by the proposed development and how it will be distributed to the transportation network.

Trip generation estimates for specific land uses are traditionally generated from rates included in most current version of a handbook entitled Trip Generation, produced by the Institute of Transportation Engineers (ITE).

Data collected to develop these rates have typically been collected at suburban, single-use, freestanding sites.  These defining characteristics limit their applicability to mixed-use development projects, which, given land use mix, design features, and setting, could include characteristics that influence travel behavior differently from typical single-use suburban developments.

Thus, traditional data and methodologies, such as ITE, would not accurately estimate the Project vehicle trip generation for mixed-use development, or developments in more dense, urban areas.

Factors that could influence trip generation include:

  • Density of development
  • Diversity of uses
  • Design (including connectivity and walkability)
  • Destinations nearby (adjacent land uses)
  • Distance to non-auto modes
  • Development scale
  • Demographic profile

If a project has any characteristics from the list above differing from those included in the data collection to develop the ITE rates, using those ITE rates almost assuredly does not take into consideration the context of the development.  As a result of the factors listed above, some trips that would other be external vehicle trips may stay internal to the site, or alternative modes may be more attractive for trips within the project site and to surrounding complementary land uses.

Other methodologies exist to develop trip generation estimates.  One such methodology was developed from a national study sponsored by the US Environmental Protection Agency (EPA).  Travel survey data was gathered from 239 mixed-use developments in six major metropolitan regions, and correlated with the characteristics of the sites and their surroundings.  Characteristics listed earlier were related statistically to trip behavior observed at the study development sites, which produced equations allowing better predictions for external vehicle trip reduction as a function of the mixed-use development characteristics.

These equations have been:

  • Approved for use by the EPA;
  • Peer-reviewed in the ASCE Journal of Urban Planning and Development;
  • Peer-reviewed in a 2012 TRB paper evaluating various smart growth trip generation methodologies;
  • Recommended by SANDAG for use on mixed-use smart growth development;
  • Promoted in an American Planning Association (APA) Planning Advisory Service (PAS), which recommended it for evaluating traffic generation of mixed-use and other forms of smart growth; and
  • Used successfully in multiple certified environmental documents.

These equations have been used for urban, suburban, and exurban mixed-use projects. Some development projects in dense, urban areas have shown reductions of 30 to 50 percent.  Mixed-use development projects in suburban areas more often show reductions between 10 and 20 percent.  Single use projects without transit connections, or mixed-use projects without complementary land uses, often show very low reductions.

Many transportation and traffic engineers will use the standard ITE rates without questioning their appropriateness.  BIG RED DOG engineers will take the time to understand your project, determine the appropriate trip generation methodology, and compile compelling documentation to support their estimates.  We have experience helping jurisdictional reviewers understanding the subtleties of the advanced methodologies as well.  At more than $3,400 per vehicle trip, it is usually well worth the investment to have traffic engineers do the due diligence on individual projects and support all trip generation assumptions, rather than using industry templates.


Do you have questions about trip generation estimates for your project? At BIG RED DOG, we’ve done this before. We’re here to help. Contact our Transportation and Traffic Engineering team today.

Written by Dan Hennessey

Dan Hennessey

Dan Hennessey, P.E., PTOE is Director of Transportation Engineering at BIG RED DOG. Dan holds a Master’s Degree in Civil Engineering (Transportation) from the University of California, Berkeley and a Bachelor of Science in Civil Engineering from The Ohio State University. Dan is responsible for managing our Raving Fans and prospective clients, overseeing the performance of our traffic and transportation engineering design staff, steering our marketing strategy, and spearheading our community outreach and volunteer efforts. Dan’s professional experience and expertise focuses on Traffic Impact Analysis (TIA) studies, highway, freeway and arterial operations analysis, signal coordination and synchronization, traffic signal design, travel demand forecasting, and pedestrian and bicycle facility design.