Traffic study finds proposed 72-unit Twisp-Carlton development would push a nearby Highway 20 intersection toward Level‑of‑Service C/D thresholds

County public works staff reviewed a traffic study for a proposed residential development south of Twisp (proposed 72 units) and summarized projected impacts to the Highway 20 / Twisp‑Carlton intersection.

The consultant’s analysis included current traffic counts and a 2027 projection (city growth at 3% plus other nearby developments). The study estimated the project would add about 538 daily trips, with a morning peak hour of roughly 45 trips and an evening peak of about 52 trips from the development. Under 2027 background growth the intersection’s worst leg was projected to see an 22.2‑second average delay without the project and about 24.5 seconds with the project — approaching the boundary between level‑of‑service C and D for an unsignalized rural intersection (Washington practice typically cites LOS C as an acceptable standard for state highways in rural areas).

Staff noted that the controlling movement and largest delay occur on the southbound leg at Glover Street rather than the direct access point from the proposed subdivision; the town is proposing to use Lombard Street for primary access, which places the direct connection within town limits and reduces county roadway impact. Commissioners were told Twisp has a funded project to study and address the Highway 20 intersection; staff said the development alone does not currently justify an immediate turn-lane requirement on the county segment but that cumulative growth could push the corridor into LOS D over time and may lead to proportional-cost discussions with developers in the future.

No development approvals were made at the meeting; staff will retain the traffic study in the application file and continue coordination with Twisp planning for access and intersection improvement timing.