TRPC modeling shows Yelm would face major gridlock during I‑5 closures; roundabouts cut delays in simulations

At a Yelm City Council meeting, Thurston Regional Planning Council (TRPC) staff presented a regional modeling study showing that a full closure of Interstate 5 would divert large volumes of traffic onto State Route 510 and State Route 507 and cause severe congestion across Yelm.

“It's not really an if. It's more like a when,” said Aidan Dixon, an associate planner with TRPC, summarizing why the detour‑route project was begun after the 2017 Amtrak derailment.

TRPC modeled full closures at three points on I‑5 and focused discussion on the segment that includes Mounts Road/Center Drive where the Amtrak derailment occurred. The simulation showed most north‑south regional flows would reroute onto SR‑510 and SR‑507 and produce major delays at dozens of local intersections. Dixon said the countywide detour network initially identified more than 50 detour routes and that the modeling helps prioritize operational and nonoperational improvements.

Nut graf: The modeling underscores Yelm’s vulnerability as a transportation pinch point—bounded by Joint Base Lewis‑McChord, Puget Sound and the Nisqually River—and suggests both short‑ and long‑term fixes. TRPC is seeking to combine operational changes (traffic geometry and intersection improvements) with communication strategies and an implementation plan to improve emergency detour performance and maintain emergency‑responder access.

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