The short answer – when a government agency tells you. The longer answer – traffic studies are typically required when:
- There is an expectation of traffic congestion anticipated with new development
- There is controversy with a neighborhood group or other citizen organization about your project
- A development needs to go through an environmental review process (such as an Environmental Assessment Worksheet – EAW, Alternative Urban Areawide Review – AUAR, or Environmental Impact Statement – EIS)
More often than not, cities will include a traffic study as part of a development’s application process. Meaning that when you propose a development, a traffic study is automatically required. Whatever the initial trigger, the resultant traffic study should fit with the size of the development. Clearly Mall of America has different study needs compared to a 30-unit residential development. So what are the different traffic study “levels”, so to speak?
It is important to find out if you are working in a jurisdiction that has published guidelines. That’s always the first step in determining the needs for your traffic study. However, many government agencies in Minnesota do not have written guidelines for traffic studies. In those instances, we rely on the Institute of Transportation Engineers (ITE). ITE has developed a Recommended Practice titled Transportation Impact Analyses for Site Development. We commonly refer to this guide in the absence of local agency guidelines.
ITE’s Recommended Practice suggests that in lieu of other locally preferred thresholds, a traffic impact study be conducted whenever a proposed development will generate 100 or more added (new) trips during any one hour period. To reach that level of expected traffic, a development would need to include more than 90 single family homes or more than a 67,000 square-foot general office building, for example.
Traffic Memo vs. Traffic Study
Are you below those thresholds? Then likely you don’t need a big study. We’ve written a lot of trip generation memos that detail the relatively minor amount of traffic expected by smaller developments, concluding that they won’t have any impact on the neighboring roadways. Developers can often use this inexpensive memo to avoid paying us for a traditional traffic study with intersection analyses.
Above expected trip generation of 100 peak hour trips, you will need a full traffic study that could include a review of nearby intersections as well as your access driveway intersections. Once again, the scope can be roughly determined by the expected trip generation of the proposed development.
The following table shows the rough breakdown of sizes and study scope based on ITE guidance.
It’s important to remember that this table is for guidance only. As mentioned, local requirements will supersede this information. But, this gives you a quick traffic study reference you can use on your projects to help you determine if a full traffic study is necessary and then ensure your development is adequately studied (without going overboard).
When you do need a traffic study, we also recommend you hire a traffic engineer who is at least familiar with ITE’s Transportation Impact Analyses for Site Development and can develop an appropriate scope of study with the reviewing government agencies.
If you are looking for a good resource on how to prepare a traffic study, check out the Traffic Study Manual at Spack Academy. This eBook does a great job of stepping the reader through all of the stages of developing a traffic study and includes a sample traffic study.