Sammamish officials defend traffic model as residents and commissioners press for more local counts and corridor analysis

City of Sammamish transportation staff and their consultant, DKS, spent the bulk of the Planning Commission’s Nov. 20 meeting explaining how the city models current and future traffic, how projects are prioritized and funded, and what the next steps are in implementing the Transportation Master Plan (TMP).

The presentation, led by Audrey Starcey (Director of Public Works) and Ben Stabler (DKS National Director of Modeling and Analytics), described a three-part toolkit: observed traffic counts, a VISUM travel-demand model for long-range forecasts and Synchro/Sidra tools for intersection-level operations. Stabler said the approach is industry standard and that calibration against observed counts produced a close match in the base year. "Modeling is one of the best ways we can do it in a reasonable, rational, structured way to do our best to estimate and, you know, drive decisions with data and analysis," he said.

Why it matters: residents and commissioners asked whether those tools capture the day-to-day experience on Sammamish roads. Several speakers produced photos of long vehicle queues at key intersections and contrasted that with model outputs that report modest per-vehicle delays that, in staff’s presentation, do not cross mitigation thresholds. "Residents feel gaslit about Sammamish traffic," said Heather Murphy Raines, a 20-year resident and member of Save Our Sammamish, calling for transparent data sources and methodology.

How the model works: Ben Stabler and the DKS team explained that observed counts are the "ground truth" used to calibrate the VISUM regional model and that Synchro or Sidra are used to analyze intersection delay and level of service (LOS). For future years, modelers convert land-use scenarios into trips, distribute and assign those trips to a network and then correct future turn volumes using NCHRP guidance derived from differences between model estimates and base-year counts.

Points of dispute and staff responses: Commissioners and members of the public pressed staff on two recurring concerns: (1) the base-year used for validation was 2023, a year still affected by pandemic-era travel patterns, and (2) the citywide or subarea modeling may average out movement-specific or corridor-level delays that motorists notice. Ajay Chakrapani and others asked how the model accounts for a potential return-to-office trend; consultants said alternative trip-generation assumptions were considered in the TMP scenarios and that models can be updated as new data arrive. When asked about the apparent mismatch between long queues seen in photographs and the model’s per-vehicle delay numbers, staff acknowledged limitations and said corridor-level microsimulation could be applied where detailed analysis is warranted, though it is costlier and usually used on a corridor-by-corridor basis rather than citywide.

Tools, thresholds and policy context: City staff emphasized that some thresholds are built into code and state guidance. Lei Wu, the city engineer, summarized the development-review sequence: "Concurrency testing is the first step in the permitting process," she said, adding that a concurrency certificate must be issued before a traffic impact fee is calculated. The presentation noted that a TIA is required for developments that generate more than 10 vehicle trips in a peak hour and that LOS standards in the TMP aim for LOS C on collectors and minor arterials and D or E for principal arterials.

Funding and timing: Audrey Starcey described how capital projects are scored and programmed in a 6-year CIP (to be extended to a 12-year constrained list as part of ongoing TMP updates) and funded primarily from Real Estate Excise Tax (REET) and Transportation Impact Fees (TIF); stormwater components are allocated from the city’s stormwater utility fund. She encouraged the public to review the updated transportation web page where the team will post the slide deck and background documents.

What’s next: Commissioners suggested regular public updates on monitoring results and raised whether performance metrics should emphasize corridor travel times (the cumulative experience) rather than only intersection-level LOS averages. Staff said they would continue annual count programs, concurrency monitoring and TIA reviews, and that targeted microsimulation or additional counts could be used where justified. The Planning Commission has scheduled a related public hearing on proposed code amendments for Dec. 4.