September 22


Traffic Impact Study Improvements: Part 4 – To the Ends of the Earth

By Mike Spack

September 22, 2015

distribution patterns, Traffic Impact Studies, traffic impact study, trip distribution, trip distribution patterns, Trip Generation, Trip Generation Traffic Impact Study, TripGen

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

Earlier this year, I detailed how our standard process for a Traffic Impact Study has several points of assumptions at best or guesses at worst. This post continues that discussion.  Check out the “Top 6 Ways to Pick Apart a Traffic Study” for more on the general topic and expect more posts to follow on this subject.

sign postTrip distribution is the process by which we take the raw projected traffic for a development (trip generation) and add it to the existing volumes on the transportation network. Which roads and routes drivers will take to and from a development? That’s the question we try to answer when we establish the trip distribution pattern for a project.

Initially, we thought we could easily test two or three different trip distribution patterns. With today’s software, it would be an easy thing to do. By analyzing a shift of five or ten percent to another direction, we could better test the capacity of a road or intersection and then determine the appropriate mitigation.

At least, theoretically.

We had our fears of producing multiple results, detailed in Part 2 of this series, which also came across clearly in your feedback on the subject. While interesting on a pure research level, a thicker actual traffic impact study report covering multiple results leads us down a path no one wants to go.

We then tried to come up with a nice formula or uniform procedure that would lead to the optimal trip distribution. But with every trip distribution methodology structure we created, it was surprisingly easy to find exceptions to each rule.

So this is a case where the art of traffic engineering applies over its science. Since we haven’t been able to develop an ironclad algorithm for determining trip distribution patterns, here are the items we consider when developing trip distribution for a project:

  1. Existing road and intersection traffic volumes
  2. Long-range projected traffic volumes from a regional transportation model
  3. Local knowledge of the area and development type
  4. Market area analysis
  5. School district boundaries
  6. Population centers
  7. Access to freeways or expressways
  8. Type of roadway facility (i.e. local residential road versus a major collector)

How to balance these factors will change with each individual project. If possible, discussing your trip distribution with a city engineer, county engineer, or other government official is also a great check and good way to achieve buy-in before completing your analysis.

Do you have a standard methodology or other factors you consider?

Did you miss the other installments of the Traffic Impact Study Improvements series? Here are the links to the other articles:

  1. Part 1 – Traffic Counts
  2. Part 2 – Would Multiple Results Help Us?
  3. Part 3 – All Trips are Equal, But Some Trips are More Equal Than Others
  4. Part 4 – 8 Considerations When Generating Trip Distribution Patterns
  5. Part 5 – When is a Trip Not a Trip? How to Determine Trip Generation Types


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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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