By Bryant Ficek, PE, PTOE
Most, and should be all, traffic engineers will readily admit our standard process for preparing Traffic Impact Studies has several points of assumptions, estimates, or flat-out guesses.
Here are the Top 6 Areas a reviewer could focus on to pick apart a Traffic Impact Study:
- Traffic Counts – we generally complete intersection turning movement counts on one day and then use that snapshot as the foundation for all of our analyses
- Trip Generation – we use the national ITE average rate for each land use, which ignores potential high/low outliers and doesn’t account for local variation or rapidly changing traffic trends
- Modal Split – many times ignored or a general reduction applied to account for transit and other modes of transportation (ITE has recently attempted to correct this)
- Pass-By/Multi-Use/Internal Trips – ITE again provides a national basis for some land uses, but the dataset is very limited
- Trip Distribution – generally based on existing travel patterns (from the intersection turning movement counts) which would not account for regional growth, development patterns in an adjacent city, or other similar types of factors
- General Background Growth – to account for non-specific growth in traffic, a percent increase is usually applied to the existing volumes, sometimes based on historic growth or a regional model
If taken to court, would we have a better defense for the above assumptions than we used “engineering judgment?”
Our tools for traffic studies have significantly improved through camera data collection and analysis programs – among others. Yet our basic process for preparing Traffic Impact Studies (and other traffic studies) has been the same for my twenty year career.
Mike and I are working through the Traffic Impact Study 2.0 – It’s time our studies became more defensible. Stay tuned…
Mike’s Take – When we review Traffic Impact Studies prepared by other consultants, we don’t pick apart every assumption they make in the study. I think it’s unprofessional to nitpick assumptions when it is one engineer’s opinion vs. another’s. Instead, we focus on intersections or corridors where the analyses indicate they’re on the edge of needing to be upgraded. In those situations, we’ll work through the assumptions to see if tweaking them will lead to a different conclusion/recommendation. And like Bryant mentioned, I think it’s time we tighten up our process and assumptions.
Want to review our installments of the Traffic Impact Study Improvements series? Here are the links to the other articles: