Traffic Volume Data Collection and Use
For any Transportation Impact Analysis (TIA) within the City of Issaquah, daily and peak hour traffic volume data must be collected at study area intersections and roadways impacted by the proposed development if the City does not already have recent counts. These traffic counts are typically conducted between 4:00–6:00 PM on a Tuesday, Wednesday, or Thursday to represent the PM peak period. The exact intersections, roadways, and time periods for data collection are established during the TIA scoping meeting with City staff.
Special considerations may require weekend or off-peak data, depending on the type and location of the development. Counts should avoid holiday weeks and ideally be collected during the school year to reflect normal conditions. If counts must be collected during school breaks, appropriate seasonal adjustments must be applied, subject to City staff approval.
Traffic data should not be collected during the following periods:
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December 15 through the first week of January
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School holidays, early dismissals, or late starts
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During or immediately after major disruptions such as crashes, road closures, or extreme weather
Counts should be broken down into 15-minute intervals and include:
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Turning movement volumes
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Peak hour factor (PHF)
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Heavy vehicle percentages
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Pedestrian and bicycle counts
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HOV lane use
Counts around schools must include data during take-in and dismissal periods. Data sources, locations, collection dates, and conditions must be documented clearly, and volumes must be summarized in graphic form.
Future Conditions and Background Traffic
Programmed Roadway Improvements
The TIA must account for committed, funded transportation projects from the City’s Capital Improvement Program (CIP), as well as improvements by other jurisdictions that may affect the study area. The report should describe each project’s scope, timing, and funding status. A map showing the location of programmed improvements should be included.
Future and Background Traffic Volumes
To assess project-related impacts, future traffic volumes must be estimated both with and without the proposed development. The City’s regional travel demand model will provide the baseline projections for the “No Build” and “Build” conditions. These projections should reflect traffic growth due to regional trends and pipeline developments.
Future traffic volumes with the development must include both background growth and project-generated trips. Graphical representations (e.g., turning movement diagrams) should illustrate daily and peak hour volumes for both future scenarios.
Project-Generated Traffic
Trip generation estimates must be based on the latest edition of the ITE Trip Generation Manual or equivalent peer-reviewed data. All assumptions, including data sources, variables, peak hour selection, and any modifications to standard trip rates, must be described in the report.
If ITE rates are based on limited data or not representative of the proposed use, a local trip generation study may be required. Any deviation from ITE rates or methodologies must be approved in advance by the Development Services Department (DSD) or Public Works Engineering (PWE) staff.
Trip credits may be applied for existing land uses that will be removed as part of the project, assuming those uses were active within the year prior to the study.
Trip Adjustments
Trip generation estimates may include the following adjustments, subject to justification and City approval:
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Internal capture reductions (for mixed-use developments): Limited to 5% unless otherwise approved
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Pass-by trip reductions (for retail uses): Limited to 15% without prior City approval
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Transit and TDM reductions: Must be supported by specific implementation plans with a high likelihood of realization within two years of project commencement
Each adjustment must be supported by documentation and empirical evidence from comparable developments. All trips, including pass-by trips, must be accounted for in the driveway and access analyses.
A trip generation table must summarize:
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Land use types and quantities
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Trip generation rates or equations used
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Adjustments (e.g., pass-by, internal capture, transit, TDM)
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Net new trips for both daily and peak hour periods
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Phased trip generation if the project is constructed in stages
Trip Distribution and Assignment
The City’s travel demand model (or a City-approved alternative method) must be used to estimate trip distribution and assignment. Any adjustments made to the model (e.g., for finer geographic detail) must be documented and approved.
Trip distribution diagrams must be included, showing directional flow by percentage and volume for Average Daily Traffic (ADT) and peak hours. Turning movements at key driveways and intersections should also be illustrated.
Traffic Operations and Impact Analysis
Three scenarios must be analyzed:
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Existing traffic conditions (without project)
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Future traffic conditions without project
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Future traffic conditions with project
The report must summarize project-related impacts on intersections and roadways and identify any locations where capacity, delay, or safety thresholds are exceeded due to the project.
Capacity and Level of Service (LOS)
Operational analysis must follow the current Highway Capacity Manual (HCM) methodologies. The City requires use of Synchro/SimTraffic Version 8.0 unless otherwise approved. City staff will provide the baseline Synchro files, which must be updated to reflect new traffic volumes and site-specific conditions.
Do not modify signal timing or phasing unless approved by DSD or PWE. Assumptions related to lane use, saturation flow, pedestrian volumes, or traffic control devices must be explicitly documented.
Intersection performance must include:
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LOS, delay, and V/C ratio for the entire intersection
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LOS, delay, and V/C for each approach and turning movement
Flag intersections where:
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Overall V/C exceeds 0.85
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Through or shared movements exceed V/C of 0.85
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Exclusive turn lanes exceed V/C of 1.0
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LOS/delay exceeds City thresholds
Where congested conditions exist (V/C > 1.0), field observations may be required to verify modeled results.
All Synchro input/output files must be submitted electronically with the report. Output tables should be consistent with the report’s summaries.
Traffic Control Warrants and Signal Coordination
If new traffic control (signals, stop signs, or yield signs) is proposed, a warrant analysis must be conducted per the Manual on Uniform Traffic Control Devices (MUTCD). Right-turn-on-red assumptions must be supported.
If a proposed signal is located near other signals, a signal progression analysis may be required to evaluate corridor-wide coordination and delay impacts.
Warrant analysis may use projected volumes, but installation of signals is contingent upon observed volumes meeting warrant criteria.
Gap Study
A gap analysis may be required for unsignalized driveways or crossings, especially when a traffic signal is requested. The study assesses whether sufficient gaps exist for safe vehicle or pedestrian movements. Special attention should be paid to vulnerable populations such as children and the elderly.
Collision Analysis
An accident history review must include five years of the most recent data for all study area intersections and road segments. This should include all types of collisions, with emphasis on those involving pedestrians or cyclists.
Summarize crashes in tabular format:
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Crash type
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Number of crashes
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Severity
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Location
Discuss any contributing roadway characteristics, such as sight distance issues, signage, lighting, or geometric deficiencies. Calculate crash rates and compare them to statewide or citywide averages:
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Intersections: Crashes per million entering vehicles
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Mid-block segments: Crashes per million vehicle miles
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