Checklist for Developing Traffic Forecasts

I’ve been a fan of checklists ever since I read Atul Gawande’s Checklist ManifestoInspired by Dr. Gawande, below is a list of things to consider when developing traffic forecasts for a Traffic Impact Study.

Checklist for Developing Traffic Forecasts

  1. No-Build, Future Year Forecasts
  • Normalize existing data to “average” weekday conditions with seasonal and weekday adjustment factors (we typically don’t do this in Minnesota because traffic is pretty consistent, instead we collect 48-hour turning movement counts to average the data)
  • Determine method for increasing existing traffic volumes
    • Growth rate based on past traffic data in area
    • Growth rate based on transportation plan forecasts
    • Based on other area traffic studies
    • Combination of the above
  1. Volumes Due to Your Development
  • Trip Generation (start with ITE Trip Generation)
    • Use average rates
    • Use best variable, such as employees or square feet
    • Use the right time frame (i.e. THE peak hour vs. the peak hour of the adjacent street traffic from 7-9 a.m.)
    • Check out our free trip generation data at TripGeneration.org.
  • Trip Generation Discounts
    • Internal trips at commercial or mixed use developments
    • Pass-by/diverted trips – ensure 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 your development, should you discount the trip generation?
  • Trip Distribution
    • Ensure you distribute the traffic per your agreed on trip distribution pattern.
    • Ensure the numbers coming in and out of your development site match the trip generation table you prepared.
  • Reasonableness Check
    • Ensure pass-by diverted trips don’t assume half the traffic on the adjacent road is going to go in and out of your development.
    • Ensure the trip generation seems reasonable for local conditions – half a small town isn’t going to visit your development every day.

Check out our Free Open Source Trip Generation Data

  1. Build, Future Year Forecasts: ensure the No-Build + Volumes Due to Development = Build Volume
  1. Round Your Forecasts to the Appropriate Level of Accuracy
  • Nearest five vehicles on low volume, peak hour movements
  • Round one and two up to five since there’s a little bit of traffic on that movement
  • Nearest ten vehicles on all other peak hour movements
  • Nearest hundred vehicles on daily volume forecasts
  • Jonathan Ehrlich says:

    Be very cautious using average ITE Trip Generation Rates. Often the averages are based on a VERY small number of studies. Look at the data points themselves. Also, the context of the data is important- ITE aggregates traffic studies together, primarily from suburban areas around the country, that are decades old.
    Why would you always round up?

  • Adam says:

    Hey Mike,
    When we consider future year forecasts, I know the standard procedure is to create a future no-build scenario which takes into account ‘background growth’ over X amount of years.
    But here’s my issue with that, isn’t our development usually part of the ‘background growth’ which will create the future year condition? If no development was built between now and the future, then background growth would be essentially zero, right? So are we double counting the trips created by the development as part of our background growth? Does background growth really happen in the background without development? Or is ‘background growth’ really more of an ‘assumed typical growth’ based on ‘typical community development patterns’? Do we then create the future traffic volumes and SUBTRACT the development to get the no-build scenario?
    Just something I’ve been mulling over for a while.

  • Jonathan Ehrlich says:

    Also looking forward to publication of NCHRP 08-66 Trip-Generation Rates for Transportation Impact Analyses of Infill Developments

  • Mike Spack says:

    Hi Jonathan – It is prudent to look at the plot of the data set, but I am also leery of saying my site will act like one other data point. If there’s a cluster of data at the unit size, going away from the average may make sense. There are pitfalls going either way. Someday I hope we can drill deeper into the ITE database to filter down to relevant data.
    Rounding up – its the conservative engineer in me. I also prefer to round 2 vehicles to 5 than to 0. 0’s really stand out.

  • Mike Spack says:

    Hi Adam – I agree that we may be double dipping on the background growth rate. I’ve been thinking a lot about getting away from “20-year forecasts” and instead giving a range of reasonable background growth rates (i.e. 0%, 20%, 40%, and 60% if you think a long term growth would be 40%). Then prepare a whole host of forecasts based on different background growth rates to talk more in probabilities than absolutes.
    Subtracting out the build forecasts from the background growth may be more honest, but spending the time to do it may not be justified.
    Mike

  • TofuDevil says:

    It is my understanding that the ITU trip generation manual is based on observed actual automobile trips. Therefore, it captures all factors of trip generation, including the fact that some people arrive by other means of transportation.
    Would it therefore be incorrect to adjust the ITU survey numbers based on assumptions that the subject proposal would vary from the ITU observation since you don’t know the non-automobile trips in the survey?

  • […] peak period counts were 8% – 46% higher than those in the traffic database. Reason? It is a common traffic engineering standard to round counts […]

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