September 13


Transportation Impact Study Guidelines

By Mike Spack

September 13, 2018

Traffic Impact Studies

When planning starts on a new or re-development project, the Traffic Impact Studies (TIS) assesses the impact on the surrounding network. A comprehensive ‘before’ and ‘after’ of traffic operations is the goal to determine if and when mitigation is needed. For many traffic engineers, the TIS is a key component of their work. This work could include the initial preparation of a TIS, the review of a completed TIS, or ensuring the mitigation identified in a TIS gets identified. Further, a traffic engineer can work for public agency, a developer, or an engineering design firm and still be involved in any and all of these phases.

Having worked on my own fair share of TIS’s across the Country (which can have a handful of other names), I recognize the importance of these documents. At the same time, we have also seen the wide variety of guidelines and policies a traffic engineer must interpret to provide a report. The simplest direction is when a town states ‘provide a traffic study’ with no other information. The level of detail to guidelines can increase sharply from there.

In our opinion, knowing the expectations of a TIS is a very good thing going into a study. Your estimates of time and fee are more accurate, which then helps developers plan their schedules and budget for the site. You can accurately plan your team’s time to completion of the document, avoiding last-minute surprises, like an added intersection. Similarly, the review can then focus on the study results as opposed to what went into the study. With the TIS completed and mitigation identified in a timely manner, the design (or redesign) of improvements is able to start or be completed quicker.

What happens without guidance? Many of us have gone through that experience too. The TIS document is prepared. The reviewer identifies extra elements to examine. One to several weeks are spent arguing of the merits of those extra elements. The Council meetings get pushed back, delaying the potential start of construction. The developer, City officials, and many others involved get frustrated. What should be a relatively smooth process turns into a bumpy road and the project that makes you sigh in frustration every time you have to work on it again.

While obviously an exaggeration, it’s easy to imagine (or remember) how quickly a good project goes south.

To avoid the nightmare scenario, we put effort into coordination at the front end of a project. This effort includes reviewing the TIS guidelines for an area, if any, and discussing the project with the public agency at the beginning, if possible. To further that coordination, we have also developed a Preliminary Traffic Assessment to present the basic traffic parameters of a proposed development and specific items we expect to review.

While we try to complete this work as part of our normal project routine, the public agency can greatly help. As we understand more about the TIS and what should go into the study, town and city policies should be reviewed and updated. Four very important items we believe need updating or inclusion in future TIS policies:

  1. Transforming the Traffic Impact Study into a Transportation Impact Study. Our industry needs to move beyond vehicles only and bring into focus transit, bicycling, and walking. How we blend these options together will impact the safety of an area and the choices people make in their daily routines.
  2. Establishing different levels of study. A small office building is different from a retail complex is different from a hundred-acre residential development. The level of study in each of those should be different.
  3. Referencing the larger planning document(s) (comprehensive plan, transportation plan, thoroughfare plan, etc.). We don’t believe the TIS should be used as a long-range planning tool. That’s what those other more comprehensive documents are for. Instead, the TIS can confirm the planning level details, identify when expected improvements are needed, or provide adjustment to the plan when conditions have changed.
  4. Allowing for communication early in the process. Formalizing a meeting at the beginning can help avoid misunderstandings and ensure the scope is set Other meetings allow for discussion of options, refinement of parameters, and generally avoid surprises.

Policies or guidance need to provide enough substance for someone to adequately prepare a study, yet flexible enough to allow for adjustments to unique situations or other specific characteristics of a site. We have helped various public agencies through this type of review. The clarity of putting all parties on the ‘same page’ is especially important in larger metropolitan areas where standards should be similar across the region, but often change based on the agency border.

In Part 2 of this series, we will examine more specifics of our vision of TIS standards.



  • The general public views Traffic Impact Studies as being dubious. There is a mathematical reason that substantiates their observations. ITE provides scatter plots, say for example distribution centers, of end trips versus square footage. Two mathematical expressions are provided, sometimes, and these are a linear regression equation and a weighted average rate. These are averages and using them results in a 50-50 chance of being right or wrong. Myers Lansky probably would have been OK with installing a few games with those odds in his establishments.

    Carl H. Buttke presented an article titled “Guidelines for using Trip Generation Rates or Equations”, ITE Journal, August 1990 wherein he discussed using confidence intervals in lieu of averages. This is a wise strategy and should be used in order to regain the general public’s trust. Township meetings where plans for a residential development or warehouse are presented can become quite heated because of the public’s distrust towards traffic engineers. Finally, the licensed traffic engineer has a responsibility to the public’s health, safety, and welfare. Lackadaisical analyses that results in allowing more traffic on the roads than would more refine analyses predicts, creates congested roads that are unsafe causing drivers to take chances that they would otherwise not take.

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    Mike Spack

    My mission is to help traffic engineers, transportation planners, and other transportation professionals improve our world.

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