SR 523 (N/NE 145th St) and I‐5 Interchange Improvements

To improve capacity and level of service at the SR 523 (N/NE 145th Street) and I-5 interchange, the City of Shoreline initiated a project to add continuous left-turn lanes across the I-5 overpass, accommodated by removing sidewalks from the overpass and constructing a separate pedestrian/bicycle bridge to the north of the existing vehicular bridge. Lochner was selected to complete an interchange justification report for the preferred alternative improvements and complete 30 percent design for the revised alternative.

Complex site conditions included the construction of a new light rail station and parking structure adjacent to the interchange on the north side, the addition of a new bus rapid transit route approaching from the east, and the construction of a new north-south pedestrian/bicycle trail immediately east of I-5. During initial analysis, the Lochner team determined the cost of the preferred alternative significantly exceeded the available budget. Using practical design guidelines, Lochner re-evaluated traffic operations and concluded the alternative’s same congestion relief goals could be achieved by constructing roundabouts at the ramp terminals, restriping the I-5 overpass for bicycle lanes, and retaining the existing sidewalks. This new design eliminated the need for a new pedestrian/bicycle bridge parallel to the existing overpass and provided the cost savings needed to move the project forward.

To help garner additional support for the new alternative, Lochner coordinated with the City of Shoreline, the City of Seattle, the Washington Department of Transportation (WSDOT), and three transit agencies to conduct a bus rodeo. The rodeo demonstrated buses could effectively maneuver through the planned roundabouts by providing a mockup of the roundabouts in a nearby bus storage lot. A technical advisory committee made up of agency representatives was also created to review the proposed modifications to interstate highway ramp terminals; as a result, all agencies accepted the Intersection Control Evaluation Report required by FHWA and WSDOT. With full consensus, Lochner completed preliminary design for the interchange’s revised alternative.