Scope of Transportation Impact Analysis (TIA) – City of Issaquah
When preparing a Transportation Impact Analysis (TIA) for a development project in the City of Issaquah, it is strongly recommended that the applicant’s traffic engineer begin the process by consulting with City staff from both the Development Services Department (DSD) and Public Works Engineering (PWE). This early coordination helps clarify the appropriate level of analysis and ensures that the methodology, assumptions, and any necessary deviations from the City’s guidelines are identified and agreed upon in advance. Taking this proactive step will help prevent unnecessary delays or future revisions during the application review process.
The scope of a TIA is determined on a case-by-case basis, tailored to the specific characteristics of the proposed development, the surrounding transportation system, and projected traffic volumes. While Issaquah provides detailed guidelines for consistency and transparency, the City retains discretion to modify or augment TIA requirements depending on unique site conditions, operational concerns, or emerging transportation issues in the project vicinity.
As an initial step, the applicant must prepare and submit a Preliminary Scoping Memorandum to the City for review. This memorandum serves as a foundation for determining whether a full TIA is required and helps City staff identify key areas of focus for the analysis. The scoping memo should include, at a minimum, the following details:
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Description of the Proposed Development and Applicable Zoning: Include a narrative summary of the project, outlining the intended land use, zoning designation, and overall development intent.
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Project Size and Type: Clearly state the number of proposed residential units, square footage of commercial or mixed-use space, and other relevant quantitative measures that define the scope of the development.
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Project Location: Provide a vicinity map showing the general location of the site in the broader transportation network, along with a detailed site plan that includes property boundaries, nearby roads, intersections, and key site features.
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Access Information: Describe proposed access points to the site, including the number and type of driveways, access to adjacent streets, and potential interactions or conflicts with surrounding properties or developments. Discuss any existing or planned access easements or shared driveways.
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Phasing and Development Schedule: If the development is to be constructed in multiple phases, the scoping memorandum must include a phasing plan. This should outline the sequencing of each phase and provide a general timeline for implementation and occupancy.
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Trip Generation Estimates: Include estimated vehicle trip generation for the development based on the most recent edition of the Institute of Transportation Engineers (ITE) Trip Generation Manual and Trip Generation Handbook. Explain any assumptions made in the calculation, such as trip reduction factors for mixed-use projects, internal capture, or transit use.
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Trip Distribution and Assignment: Present the expected distribution of project trips across access driveways and the local roadway network. This includes percentages assigned to specific directions and how trips are allocated to intersections or access points. A visual diagram may be helpful to illustrate trip paths.
Once the City receives and reviews the scoping memorandum, it will determine whether additional analyses are warranted. If a full TIA is deemed necessary, City staff will provide the applicant with relevant background data and scoping parameters to support the study. This may include:
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Traffic Count Data: The most recent available counts for intersections and road segments within the study area.
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Collision and Safety Data: Information on crash history and designated High Accident Locations (HALs) that may be affected by the proposed development.
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Existing Synchro Files: Capacity analysis model files, if available, which applicants can use as a starting point for their own modeling efforts.
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Planned Improvements: A list of funded or programmed improvements (motorized and non-motorized) in the vicinity of the project, including roadway widening, pedestrian crossings, bike lanes, or signal modifications.
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Pipeline Projects and Background Growth: Data and assumptions used for projecting future background traffic volumes, including developments in the pipeline and city-wide growth assumptions.
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Special Studies Requirements: City staff may identify the need for additional studies such as sight distance evaluations, queuing analysis, channelization warrants, or pedestrian and bicycle impact assessments.
After review of the scoping memorandum and supporting documents, the City may require a TIA Scoping Meeting with the applicant’s traffic engineer and relevant City staff to further define the study area and clarify any technical or procedural issues. The purpose of this meeting is to ensure that all parties agree on the assumptions, geographic scope, intersections of concern, data collection needs, and analytical methodologies before significant work on the TIA begins.
Additionally, if other agencies are impacted by the development—such as the Washington State Department of Transportation (WSDOT), King County, or the City of Sammamish—the applicant may be required to coordinate with those jurisdictions. In these cases, the City may request that representatives from all affected agencies participate in the scoping meeting. This helps streamline the review process, improves interagency coordination, and reduces the likelihood of late-stage changes or conflicting recommendations.
The City reserves the right to adjust or expand the study area based on its understanding of the local transportation network, projected traffic impacts, or public safety concerns. Applicants should be prepared to justify any assumptions or methodological choices, and must incorporate all agreed-upon elements into the final scope of the TIA.
By establishing the scope through a collaborative and transparent process, the City of Issaquah seeks to ensure that Transportation Impact Analyses are thorough, consistent, and responsive to both community needs and project realities.
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