I-5 - 179th St. Interchange - Interchange Improvements

Needs & benefits

This project is funded as part of the Connecting Washington program and is being planned in partnership with Clark County as one of a series of projects along the Northeast 179th corridor. These transportation improvements will support the Clark County Comprehensive Growth Management Plan for the area. See the Clark County NE 179th St Corridor Improvement project webpage for more information on the projects being planned by the county and private development.

These transportation system improvements are necessary to support the future growth and development in Clark County. More importantly, this project will improve traffic flow and reduce delays that will occur if nothing is built. 

FAQ's

“Why roundabouts?”
Roundabouts are an effective intersection type because they allow for improved traffic flow and have safety benefits.  Besides having fewer conflict points, they have a traffic-calming effect because the design of the roundabout has a series of curves that force drivers to slow down as they approach the roundabout rather than relying solely on traffic control devices. They also require less maintenance and are cost-effective.

“Were other interchange types studied?”
Yes! Clark County and WSDOT have studied over a dozen interchange concepts over the years. This included other types such as signals, roundabouts, diverging diamond and single point interchange concepts with different layouts. We also presented some of these options to the public during various community outreach events.

“Will this accommodate event traffic and growth in the area?”
Future growth in the area will increase traffic at this interchange. The purpose of this project is to address and better accommodate this growth and improve traffic flow.

A traffic study was completed and determined that roundabouts will best serve traffic volumes from both current and future development in the area. Event traffic was also accounted for in the traffic study and was best accommodated with a roundabout interchange.  Like today’s practice, manual traffic control such as flagging, will be used to manage event traffic, as it would be with any interchange type.

“Will trucks be able to use the roundabout?”
Yes! Roundabouts are designed for location-specific needs and can accommodate the types of vehicles that are anticipated to use this interchange. WSDOT will work with nearby freight companies to tweak the roundabout design to accommodate their needs. Here are some examples of truck-friendly elements that can be included in a roundabout design:

• A mountable, flat central island.
• Extra low curbing height.
• Removable signing.

“This project may include a “truck apron,” what is that?”
A truck apron is an intentionally-designed element of a roundabout where a raised section of concrete around the center island of the roundabout acts as an extra lane for large vehicles. The back wheels of the oversize vehicle can ride up on the truck apron so the truck can easily complete the turn. The truck apron is raised, and often a different color or material than the roadway, to discourage use by smaller vehicles and emphasize that it is not a normal travel lane.

“Don’t roundabouts create more congestion?”
Roundabouts move traffic through an intersection quickly and efficiently, and with less congestion on approaching roads. Roundabouts promote a continuous flow of traffic. Unlike intersections with traffic signals, drivers don’t have to wait for a green light at a roundabout to get through the intersection. Traffic is not required to stop – only yield – so the intersection can handle more traffic in the same amount of time.

Studies by Kansas State University measured traffic flow at intersections before and after conversion to roundabouts. In each case, installing a roundabout led to a 20 percent reduction in delays. Additional studies by the Insurance Institute for Highway Safety (IIHS) of intersections in three states, including Washington, found that roundabouts contributed to an 89 percent reduction in delays and 56 percent reduction in vehicle stops and studies by the Federal Highway Administration (FHWA) show roundabouts can increase traffic capacity by 30 to 50 percent compared to traditional intersections.

“Roundabouts are confusing!”
WSDOT understands that learning how to move in new traffic patterns can be an adjustment. However, we have seen time and time again that after a roundabout is constructed, the community quickly learns and becomes more comfortable navigating the roundabout. If you are new to driving through roundabouts, learn how to navigate a roundabout here.

“Did WSDOT do public engagement on this project?”
Yes! Beginning in 2019, after the Legislature provided early funding to do project planning, the WSDOT project team shared project information on possible interchange types and collected public input at various community outreach events. We will host public open houses in 2024 and 2025 to continue sharing updates and engage with critical roadway users, such as freight, pedestrians and bicyclists, to ensure the roundabout design will accommodate all vehicles that travel through the intersection.

“Are more roundabouts going to be installed in Southwest Washington?”
It’s true! Roundabouts are here to stay as a tool in the toolbox to address safety and efficiency, but they may not be the ideal solution for every intersection. Several factors are evaluated when deciding to build a roundabout at a specific intersection. Engineers consider the following characteristics when determining the best solution for a particular intersection:

• Accident history. Data about the number of accidents, type of collision, speeds, and other contributing factors are analyzed.
• Intersection operation. The amount of current and future traffic, and projected travel delays and backups on each leg of the intersection are examined.
• Types of vehicles using the intersection. It is important to look at all the different kinds of vehicles that use the intersection. This is especially important for intersections frequently used by large trucks.
• Bicycle and pedestrian needs.  It is important to understand the level of anticipated bicycle and pedestrian traffic, and associated travel patterns.
• Cost. This includes not only the basic costs of construction, but also social and economic costs, right-of-way (buying land) requirements, and long-term maintenance expenses.