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.

Most important findings

- Concentration of diverted traffic: The model shows large increases in traffic on SR‑510 through Yelm and on Rainier Road as drivers attempt to bypass I‑5. TRPC said impacts would be felt across the county but concentrate in the Yelm corridor.

- Roundabouts reduce delay in model runs: TRPC simulated installing a series of roundabouts at key intersections around Yelm and compared closure scenarios with and without roundabouts. In one example the model estimated average vehicle delay at a key intersection of roughly 310 seconds without a roundabout versus about 12–14 seconds with a roundabout during the modeled closure period. Dixon cautioned the results are model estimates and “shouldn't really be taken as gospel,” but called them useful for comparing options.

- Freight and truck issues: TRPC emphasized truck mobility and parking as critical concerns. The study found truck parking would likely be insufficient during extended closures and noted special considerations for heavy commercial traffic that moves between Seattle and Portland and services distribution centers near JBLM.

- Vertical clearances and GPS deviations: While TRPC said the official detour network avoids low clearances, staff found nearby parallel roads not on the official network with low bridges (examples cited in discussion: 14 feet 3 inches and 12 feet 6 inches). That raises a risk that GPS routing or drivers deviating from official routes could send large trucks onto unsuitable roads.

Questions, recommended mitigation and next steps

Council members asked practical measures. Councilman Trevor Palmer, drawing on commercial driving experience, urged clearer signage for truck routes and for warning signs at off‑route low bridges so out‑of‑area or infrequent drivers do not follow inapt shortcuts. Councilor Marquez suggested TRPC pursue contact with major GPS vendors to update truck‑specific routing data; TRPC said it had not formalized partnerships with Google or Apple but planned outreach.

On timing, Dixon said TRPC received a Federal Highway Administration surface transportation block grant (administered by WSDOT) in 2022 to support the work. TRPC plans to issue an RFP for an operational analysis in spring, onboard consultants in summer, produce a resiliency (nonoperational) strategy and then combine findings into an implementation/action plan next year. Mayor and council interest accelerated that RFP timeline, staff said.

Other technical and coordination issues noted

- Cross‑jurisdiction communication: TRPC emphasized the need to formalize communications and chain‑of‑command protocols between Thurston and Pierce counties, WSDOT, Washington State Patrol and local law enforcement to avoid delays in response and traffic management.

- Emergency responder access: Council members and TRPC staff discussed how dense detour traffic could impede EMT and law‑enforcement access to incident sites and noted that any plan must prioritize clear access routes for emergency vehicles.

- Truck parking and staging: The study flagged limited truck parking options during extended incidents and noted existing rest areas and Fort Lewis parking may provide partial relief depending on timing.

Ending: TRPC will return with the consultant analysis and an implementation plan when work is complete. Dixon said a current‑conditions report with intersection‑level detail will be available for local review soon; TRPC staff committed to follow up on signage and outreach to GPS vendors and to coordinate the RFP schedule with city staff.

Votes at a glance

- Consent agenda (approval of minutes for 02/04/2025 and 02/11/2025): Motion to approve made by Councilor Kaminski; second recorded as “Councilor S.” Outcome: approved (voice vote).

- Appointment to Lodging Tax Advisory Committee: Motion to approve Mayor's appointment of Amar Darwaul as a lodging‑tax‑qualified hotel representative. Mover: Councilor Haas. Outcome: approved (voice vote).

- Adjourn: Motion to adjourn by Councilor Crossman. Outcome: approved (voice vote).

All quotes and attributions above come from Yelm City Council meeting remarks and the TRPC presentation recorded in the meeting transcript.