September 29


How to Prepare Traffic Forecasts for Traffic Impact Studies

By Mike Spack

September 29, 2011

Traffic Impact Studies

I attended the NCITE (North Central Section of ITE) planning methods committee meeting because traffic forecasting in traffic impact studies was on the agenda.  There was a good discussion, but not a lot of conclusions.  Below is a checklist that details out how we prepare our traffic forecasts.

  • No-Build, Future Year Forecasts (Determine method for factoring up existing traffic volumes)
    • Typically use Mn/DOT’s assigned State Aid growth factor that’s been assigned for the County
    • Growth rate based on past traffic data in area
    • Growth rate backed out from transportation plan forecasts
    • Based on other area traffic studies
    • Combination of the above
  • Volumes Due to the Development
    • Trip Generation (start with ITE Trip Generation)
      • Use average rates
      • Use best variable (i.e. employees or square feet?)
      • Make sure you’re using the right time frame (i.e. THE peak hour vs. the peak hour of the adjacent street traffic between 7-9 a.m.)
    • Trip Generation Discounts
      • Internal trips at commercial or mixed use developments
      • Pass-by/diverted trips – make sure they aren’t a large portion of the road’s existing traffic (rule of thumb – limit to 10% of existing traffic)
      • If there is heavy transit use or in a dense urban neighborhood, should you reduce the trip generation to account for alternate modes of transportation?
      • If you’re implementing a Travel Demand Management Plan for the development, should you discount the trip generation?
    • Trip Distribution
      • Make sure you distribute the traffic per your agreed upon trip distribution pattern.
      • Make sure the numbers coming in and out of the development site match the trip generation table you prepared.
  • Build, Future Year Forecasts:  make sure the No-Build + Volumes Due to Development = Build Volumes
  • Round your forecasts to the appropriate level of accuracy (round up)
    • Nearest 5 vehicles on low volume, peak hour movements
    • Nearest 10 vehicles on all other peak hour movements
    • Nearest 100 vehicles on daily volume forecasts
  • Make sure the volumes going between adjacent intersections add up on all Volume Due to Development forecasts and between adjacent intersections in the No-Build and Build forecasts if there are no intersections between them.  Cars don’t levitate.
  • It seems to me that traffic impact studies that accurately measure existing LOS in a peak hour, peak direction are important. Especially because that LOS (actually its associated volume count) is used as a base volume for trip generation studies of a proposed development. But, the volume counts used to obtain LOS from the Generalized Service Volume Tables result in a gross misrepresentation of LOS when a roadway is already congested. This is because as traffic slows due to congestion, the hourly count actually decreases as traffic gets worse – resulting in a falsely low volume measure of traffic. So, a severely overburdened roadway gets a LOS B or C, when it is really (by Average Travel Speed measure) a E or F. This is a common occurrence here in South Florida impact studies. And when these flawed studies are accepted as truthful at public hearings, congested roadways are being burdened with even more trips that degrade or further degrade LOS well beyond the legal standards.

    Use of a Average Travel Speed method to measure LOS appears to be accurate, but it does not provide a base volume to which trips generated by a proposed development will be added. How does one get an accurate volume count to use as a base for trip generation on an already severely congested roadway? Without an accurate base volume, developers’ flawed traffic studies using the Generalized Service Volume Tables are being accepted with the result that developments are being approved without adequate infrastructure to support them, or mitigation to pay for infrastructure improvements.


  • Hi Stephen – I don’t have any easy answers for you. Along with development comes traffic. It is up to each jurisdiction to define the appropriate balance for their community and then how they will measure that balance. Mike

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    Mike Spack

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