PLANNING AND ZONING COMMISSION AGENDA ITEM
ACTION REQUESTED:
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Receive the report on the traffic impact study process within Naperville
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DEPARTMENT: Transportation, Engineering and Development
SUBMITTED BY: Andrew Hynes, PE, PTOE, City Engineer/Deputy Director
ENTITLEMENTS REQUESTED:
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BOARD/COMMISSION REVIEW:
N/A
BACKGROUND:
A Traffic Impact Study (TIS) is a site-specific study conducted to determine the impact of the traffic generated by a proposed development on the surrounding roadway network. The study analyzes the roadway network for the pre- and post-development scenarios to determine how the development impacts the operations of the roadway network. Traffic Impact Studies typically contain the following elements:
• Existing traffic data (collection of peak hour traffic volumes and movements)
• Trips Generation - from the proposed developed (typically developed using the Institute for Transportation Engineers Trip Generation Report or case studies of similar developments)
• Trip Distribution - Assigning the direction distribution of existing and entering new trips
• Capacity Analysis - Evaluating the before and after traffic conditions using established traffic engineering principles and software tools.
• Conclusion and Recommendations - If the projected traffic impacts are significant, site-specific mitigation measures such as new turn lanes or traffic signal timing adjustments are recommended.
A TIS evaluates the site-specific impact that a development has on the Level of Service (LOS) of an intersection or a roadway. The LOS is a grade from A to F where Level A indicates free flowing conditions without delay, and Level F indicates high levels of congestion, and most motorists do not get through an intersection in one traffic signal cycle. The City desires a LOS D or better for an intersection during the peak travel hours except for downtown Naperville where a LOS E is considered adequate.
DISCUSSION:
When evaluating the need for a TIS, the city looks at 3 scenarios:
1. The proposed development is not consistent with the City’s Land Use Master Plan.
2. The City and developer disagree regarding geometric improvements.
3. The proposed development is anticipated to involve intense public, Planning and Zoning Commission, and/or City Council scrutiny regarding traffic issues.
The third scenario is not always easily identifiable. The need for a traffic study is discussed by the Engineering Services Team during the concept stage of the development process to determine what level of traffic evaluation is appropriate.
Arterial Network Analysis and Improvements
In addition to evaluating site-specific impacts from development through a traffic impact study, the City has a Road Improvement Plan that considers the arterial road network needs based on future regional development. The current Road Improvement Plan was reviewed and approved by the Transportation Advisory Board and City Council in 2025. A copy of the Road Improvement Plan is attached.
This plan identifies the existing and future operational conditions of intersections and road corridors within Naperville. Based on this analysis, capacity improvements (additional through or turn lanes) are recommended based projected 2050 traffic conditions. These improvements (for streets under Naperville’s jurisdiction) are then programmed in the City’s capital improvement budget for implementation over the next 20+ years.
Future traffic volumes were developed using the Chicago Metropolitan Agency for Planning’s (CMAP) On To 2050 travel demand model. This model considers the anticipated growth in trip generation from new development and other transportation related factors for Naperville and surrounding communities.
Other area transportation agencies, such as the Illinois Department of Transportation (IDOT), DuPage County, and Will County, also require traffic impact studies when developments occur adjacent to their roadways. See the attached Roadway Jurisdiction Map for reference. These agencies program larger capital improvements on their transportation network to address regional traffic growth as well.