The Tulalip Tribes, WSDOT, FHWA, along with stakeholders in Snohomish County, City of Marysville and Community Transit went through an extensive process of investigating the existing traffic conditions, and developing new interchange and local road improvement alternatives before evaluating, screening and selecting the preferred alternative to improve the interchange.

Traffic problems identified at the interchange included;

  • Signals on 116th Street NE west of the interchange were to close together
  • There are not enough thru lanes eastbound and westbound on 116th Street NE thru the interchange to handle the traffic
  • The off ramps from I-5 to 116th Street NE don’t have enough storage
    length to hold the number of cars exiting thereby backing up stopped traffic on to I-5.
  • Accident rates are much higher than the statewide averages and are attributed to congestion and gridlock.

WSDOT has established LOS D as its acceptable level of service (LOS) on highways of state-wide significance, such as I-5. Analysis of the LOS on the 116th Interchange shows that the interchange currently operates at the threshold to LOS E for the critical approaches, with long queue lengths and high accident rates. The existing northbound ramp intersection, which is the critical intersection of the interchange, carries approximately 2,500 PM peak-hour trips (25,000 average daily trips [ADT]). Additional development in the vicinity will push the level of service to unacceptable LOS E, with queues extending into the I-5 mainline and increased accidents.

Several alternatives were looked at and a Single Point Urban Interchange (SPUI) was selected. Key project elements include widening of all interchange ramps, HOV bypass lanes, ramp metering, replacement of the existing overpass with a wider bridge, bike lanes and sidewalks across I-5, and local road improvements to improve interchange operations. The local road improvements include:

  • Removal of the signal at 34th Avenue NE ant 116th Street NE Intersection
  • Restricting 34th Avenue NE to right in right out access to and from 116th Street NE
  • Extending Quil Ceda Boulevard north of 116th Street NE to intersect with 34th Avenue NE
  • Installing a roundabout at the extension of Quil Ceda Boulevard connection with 34th Avenue NE ¾ of a mile north of 116th Street NE
  • Adding eastbound and westbound lanes to 116th Street NE between the Quil Ceda Boulevard and the east side of the interchange
  • Adding a left turn lane from westbound 116th Street NE in to the truck stop west of the interchange
  • Adding additional left and right turn lanes on all four legs of the 116th Street NE and Quil Ceda Boulevard intersection

Without improving the interchange, in 2042, the average vehicle delay during peak hours will exceed five minutes. With the interchange improvements, overall peak hour average vehicle delay will drop to 30 seconds.

A SPUI operates at one signal rather than the traditional two signal diamond interchange that exists there today. The SPUI signal operates in three light phases.