December 16


Traffic Impact Study Improvements – Round-Up

By Mike Spack

December 16, 2015

TIS, traffic impact study

Guest Post by Bryant Ficek, PE, PTOE, Vice President at Spack Consulting.

TIS Improvment RoundupWe have completed and/or reviewed thousands of Traffic Impact Studies (TIS) over the 15-year history of Spack Consulting. And while the technology continues to make improvements, the standard TIS has remained about the same. Being the curious engineers that we are, we started looking at the biggest holes in our process and whether we can make them better.

To review, here are the top 6 issues we find with a general TIS:

  1. Only one data point of traffic counts
  2. Unknown basis of trip generation data
  3. Modal splits
  4. Pass-by, multi-use, and internal splits
  5. Trip distribution to the roadway system
  6. Generic background growth

Through our review of the generic TIS, the comments you provided on this website, and the input of other professionals around the country, we think we have developed some great strategies to implement that will improve our studies without adding significantly to a project’s cost or schedule.

  • Obtain two consecutive days of turning movement counts and use an average, potentially with a seasonal factor, to determine the base data for your study.
  • Obtain or provide your own local trip generation data to better reflect your proposed development (see org for our start of open-source data).
  • Consider multiple factors/characteristics when determining trip distribution patterns and, if possible, discuss with the reviewing agency.
  • Use the latest ITE information for modal splits and trip type splits as well as be able to justify deviations. If possible, discuss with the reviewing agency.

With the gains in inefficiencies from using video technology for traffic counts and using VISTRO/VISSIM for analysis, we have implemented these takeaways without increasing our TIS prices. Our studies have also become more scientific resulting in a better planning tool for governing agencies. Hopefully, this has resulted in better use of our limited resources on the transportation system as well.

Want to review our installments of the Traffic Impact Study Improvements series? Here are the links to the other articles:

Intro – Top 6 Ways to Discredit a Traffic Study

Part 1 – Traffic Counts

Part 2 – Would Multiple Results Help Us?

Part 3 – All Trips are Equal, But Some Trips are More Equal Than Others

Part 4 – 8 Considerations When Generating Trip Distribution Patterns

Part 5 – When is a Trip Not a Trip? How to Determine Trip Generation Types

Part 6 – Should Safety Review be Included in a TIS?

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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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