The City of Issaquah has established detailed requirements for the preparation and review of Transportation Impact Analyses (TIAs) to ensure that development proposals maintain the safety and functionality of the city’s transportation network. Administered by the Development Services Department (DSD) and Public Works Engineering (PWE), these requirements are designed to assess probable significant adverse impacts related to roadway operations, safety, non-motorized transportation, and other mobility considerations. TIAs must address localized and site-specific impacts even if the development complies with system-wide concurrency requirements.
Intersection Level-of-Service (LOS)
The City's standard for intersection performance is Level of Service D, as defined in the most recent Highway Capacity Manual. If a proposed development causes a roadway or driveway—particularly one not included in the City's concurrency network—to fall below LOS D, it is deemed to have a probable significant adverse impact. Mitigation measures must be identified and implemented to restore acceptable operations. In addition to City standards, developments affecting roads under other jurisdictions (e.g., WSDOT, King County, Sammamish) must adhere to their LOS standards as well.
Safety Impacts
Safety is a critical element in determining significant impacts. If a development adds ten or more peak-hour trips to a High Accident Location (HAL), it is presumed to have a significant adverse effect. Even if LOS remains acceptable, mitigation may be required. Safety evaluations also include considerations like sight distance and turning movements. In cases where a HAL has an already programmed improvement project, developers may contribute financially as part of the mitigation.
Channelization and Traffic Control Devices
If a development’s traffic volume necessitates channelization improvements—such as turn lanes or acceleration/deceleration lanes—at local, non-concurrency intersections or access points, it will be considered to have a significant impact. Similarly, requests to install or modify traffic control devices, such as new traffic signals or four-way stop controls at intersections serving the development, also constitute a significant impact and trigger a requirement for a TIA and mitigation planning.
Non-Motorized Transportation
The City places a strong emphasis on pedestrian and bicycle safety and mobility. A significant adverse impact occurs if a project:
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Creates new conflicts for pedestrians or cyclists,
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Fails to meet ADA (Americans with Disabilities Act) standards,
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Does not ensure sufficient pedestrian or bicycle access,
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Worsens existing unsafe conditions for non-motorized users.
Transit and Site Circulation
Transit impacts are deemed significant if a development increases transit demand beyond what can be reasonably provided by the local transit operator. Site and internal circulation issues are also evaluated, including:
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Conflicts at access points with public traffic flow,
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Poor internal circulation for vehicles or pedestrians,
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Inadequate accommodations for delivery or service vehicles that could disrupt traffic.
Parking Impacts
A development is considered to have a significant parking impact if projected demand exceeds the supply regularly or if it leads to the frequent use of neighborhood parking spaces intended for residents. Such issues may require redesign or the inclusion of demand management strategies.
Submission and Review Process
TIAs must be prepared by a Professional Civil Engineer registered in the State of Washington, with specialized experience in traffic and transportation engineering. Submissions to the City must follow a three-step process:
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Preliminary Scoping Memorandum – This must be submitted prior to data collection and traffic counts. It defines the scope, methodology, and key assumptions to be used.
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Draft Report – Based on the scoping memorandum, this includes data analysis, forecasts, and recommended mitigation strategies. A thorough review will be conducted by City staff.
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Final Report – This version must incorporate all comments and revisions resulting from the City's review. A response letter outlining how feedback was addressed must be included. Final documents must bear the professional seal of the engineer responsible for the study.
TIAs must use Synchro/SimTraffic version 8.0 or other City-approved traffic modeling software. Appendix C of the City’s guidelines outlines the required assumptions, factors, and modeling methodologies. At the time of submission, the developer must provide:
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Two printed copies of the draft and final report (including appendices),
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One digital copy containing all data and analysis files,
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Additional copies if other jurisdictions are part of the review process.
If multiple jurisdictions are impacted, the traffic consultant must schedule a scoping meeting with relevant agencies (e.g., WSDOT, King County). This collaboration minimizes the need for future revisions and ensures all agencies' concerns are addressed. The City requires correspondence with external agencies to be submitted as part of the review process.
TIAs may be returned for revisions based on data accuracy, completeness, impact evaluations, compatibility with site access plans, or other factors identified during the review. Revisions are also required if:
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The land use proposal is changed in type or size,
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The TIA is more than two years old and the project has not advanced through approvals.
The final, consolidated TIA must integrate all revisions and responses to City feedback. It becomes a public document and can be referenced in future submittals. Applicants are expected to cite original sources when using prior data or analysis. Once submitted, the TIA helps the City make informed decisions that balance growth with transportation system integrity.
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