The City of Issaquah’s Development Services Department (DSD) and Public Works Engineering (PWE) have created comprehensive guidelines for preparing Transportation Impact Analyses (TIA), with a focus on both motorized and non-motorized modes of transportation. These guidelines support the City's transportation concurrency and multi-modal impact fee framework, ensuring that new developments are assessed for their impact on the existing transportation infrastructure in a timely and consistent manner. They are subject to change by the City as needed to reflect evolving transportation needs and policies.
A TIA is primarily conducted to evaluate the impacts a proposed land use development may have on the surrounding transportation network. It helps determine whether the existing infrastructure can accommodate the new development without compromising operational efficiency or safety. The analysis also aids in identifying necessary transportation improvements to meet the City’s operational standards, ensuring safe and effective access to the site, and pinpointing any current or future deficiencies within the system. Importantly, TIAs guide decision-makers by outlining the transportation-related implications of development proposals and help in determining appropriate mitigation costs and the developer’s share of those improvements.
A TIA is generally required when a proposed development is expected to generate 30 or more peak hour vehicle trips. However, the City may also mandate a TIA for smaller developments if traffic-related concerns—such as proximity to high-accident locations or areas with known congestion—warrant a detailed analysis. Additional triggers for requiring a TIA include zoning changes, new building permits, site plan applications or amendments, subdivision or platting applications, new driveway access points, comprehensive plan amendments initiated by developers, or requests for new traffic signals.
Developers must begin with a preliminary trip generation assessment, typically submitted with the Transportation Concurrency Application. The City uses this assessment to determine whether a full TIA is needed. Issaquah has already completed a city-wide concurrency assessment, including trip banks and transportation impact fees based on planned growth. Therefore, individual applicants are not required to conduct system-wide intersection capacity analyses if their proposal aligns with the City’s growth plans and they pay the applicable impact fees.
However, localized impacts must still be assessed. These may include access management, non-motorized transportation impacts (e.g., pedestrian and bicycle facilities), geometric design, safety concerns, transit access, and potential neighborhood impacts. Once the concurrency assessment is complete, developers should consult with DSD staff to confirm whether a local TIA is needed.
A TIA might be waived if the project is part of a larger development for which a traffic study has already been accepted by the City. It is the applicant’s responsibility to demonstrate that their project’s impacts are consistent with that study. The City will make the final judgment on whether the existing analysis is adequate.
Ultimately, a TIA is a critical tool for preserving traffic flow, enhancing public safety, supporting sustainable development, and ensuring infrastructure keeps pace with growth. The type and timing of necessary improvements are determined based on multiple criteria such as level of service (LOS), vehicle delay, queue lengths, safety analysis, and impacts on various transportation users and modes.
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