TIPS FOR UNDERSTANDING A TRAFFIC IMPACT STUDY


Traffic Impact Studies are required for many development projects and can range in size from a one-page worksheet to a one-thousand-page report. They serve a key role in showing how the existing roadway infrastructure is operating and how a proposed development is going to impact the study network. But let’s face it – traffic impact studies aren’t exactly page turners. They are full of technical information that can be confusing and tedious to read. The good news is that, for the most part, all traffic studies are set up very similarly with few exceptions. Knowing what to look for as you read will help you make sense of the data and consider the recommendations.


Trip Generation and Distribution The trip generation and distribution portion of the traffic study determines the number of vehicle trips that are anticipated to access a development. The major point of focus in this section is the net new peak hour trips, typically in the afternoon/evening. Depending on the size of the development and land-uses, there can be everything from total trips, daily trips, internal capture trips, and pass-by trips. For most users, the number that is going to have a direct effect on your commute is the AM/PM net peak hour trips. These are the new vehicles (after all reductions) that will be added to the roadway network and are going to directly impact your commute times. Additionally, pay attention to the distribution of those vehicle trips, as this will tell you where the vehicles are anticipated to be coming from/going to and will give you a better sense of how it will impact the roadway network.


Level of Service Throughout the report there will be many references to Level of Service (LOS). LOS is a ranking system for vehicle delay in seconds and traffic flow. Without getting into a detailed discussion on LOS, it generally boils down to LOS A, B, C are good and mean the network is running smoothly. LOS D means the network is starting to slow down, but for peak hours, LOS D is generally acceptable. LOS E can be acceptable in certain cases, and LOS F is typically unacceptable. As always, there are exceptions to the rules and certain roadways or intersections can’t be improved without significant changes outside the scope of a development project. If you see LOS E/F and no recommendations for improvements, it should either be explained in the report, or there is an exception in the development codes.


Existing/Background Conditions Existing and background conditions set the baseline for the study network. In this section, we want to see if the network has pre-existing issues, such as failing intersections, roadway segments over capacity, or deficiencies in roadway geometry. Based on these issues, the developer may not be required to fix what is already broken, especially if they are not having a significant impact to the study area or they are not impacting the problem movements.


Build Conditions Build conditions represent what the anticipated future study network will experience. In these sections, we want to focus on any negative changes from the existing or background conditions, such as LOS degrading from LOS C to E or F, or a LOS F going from a 70 second delay to a 1,000 second delay. If there are no or minimal changes, then the development is not going to impact the roadway network significantly. If there are changes, then we consider how bad the development will make the roadway network. A change from LOS D to E might look bad, but if delay is only changing by one second, then there is not much of a problem. Required mitigation is typically based on the analysis of these results.


Required Mitigation / Conclusions / Recommendations Typically, the required mitigation will be summarized at the end of the report and should identify the problem and what will be required to fix it. Ideally, any existing or background problems should be outlined here as well, but that is unfortunately not always the case. If you don’t trust the recommendations, go back and review the build conditions and alternatives analysis to determine if there actually is a problem with the study network, and why or why not the improvement is needed.

Don’t let traffic studies intimidate you. This simple primer should help you make sense of the data and understand how your development will impact the surrounding traffic network.


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