Breaking ground on the 405/167 HOV direct connection last year
Most Renton residents are aware that a major construction project is occurring at the intersection of I-167 and I-405, bringing much-needed improvement to this overwhelmed intersection. But what is less well known is how the current project, the connection of the HOV lanes from I-405 to I-167, will fit into a larger project that may not be completed for another decade or two depending on when funding is available.
I’ve prepared this blog entry to help clarify the situation.
The original I-405/I-167 interchange makes use of four “clover leaves” to transfer motorists from one roadbed to another. It may be an appropriate design today for the crossing of two rural highways in a sparsely populated area. It was not too bad in 1965 when it went into service. But this design requires cars to slow to about 20-25 miles an hour to make the curves of the clover leaves and is not an effective design for a modern urban freeway connection that is moving tens of thousands of motorists per hour. As motorists slow to make the cloverleaf turns, they quickly form a queue that backs up onto the freeway, bringing traffic to stop-and-go conditions in which the road loses two-thirds of it’s carrying capacity.
The proper fix for this is to keep motorists moving at 45-50 miles per hour as they make the freeway connections, keeping traffic at it’s optimum carrying capacity. (Freeways carry the most cars per hour when motorists are moving at a steady 50 miles per hour or so; at lower speeds people will follow so close they can trigger a stop-and-go condition, which dramatically cuts carrying capacity, and at higher speeds like 65-70 good drivers will spread out so much for safety that the capacity drops.)
One of the challenges with making these high-speed connection ramps at the 405/167 intersection is that we have important Renton local access to the freeway at this location. Rainier Avenue/405 is used by tens of thousands of Renton motorists per day to get access to I-405 and 167. We are trying to join two major urban freeways and serve local residents all at the same location.
So the State Department of Transportation has been working with Renton to fix freeway flow AND preserve our local access to I-405. A solution has been identified. The final configuration of the new freeway is not funded and is not widely advertised, so I am sharing it here.
The final configuration, which I don’t expect to see for maybe another 10-15 years or so, will give Renton residents I-405 access via a “split diamond” interchange system, offering on-ramps to northbound I-405 via Talbot Road (a project we completed about 7 years ago), and access to southbound I-405 via Lind Avenue (a project that will mirror the Talbot Road interchange and be completed in the future.) The Talbot and Lind Avenue onramps will be connected to one-another via improved frontage roads. This final arrangement will eliminate three of the cloverleaves on I-405/ I 167, leaving only the leaf that handles the relatively low volume of Northbound 167 drivers merging onto South/West I 405.
The project in work now will not complete all of this work, but will be an important component that will add near-term relief. It has been engineered to support the final configuration.
The above project is the one in-work now (for which we held the ground-breaking above). The flyover ramp shown in the picture connects the HOV lanes from Northbound and Southbound 405 and 167. You can read more about this project here
The future final configuration will add direct connections for single occupant vehicles, and will look like the picture below. We will need to work with the state and federal government to secure more funding to complete this.
Future (indefinite date) completed I405/I167 interchange
Engineering layout of proposed new “split diamond” interchange, providing local access to I-405 from Lind Avenue and Talbot.
If you are interested in learning more about the final plan, you can go to this link.
Keeping I 405 and I-167 flowing is extremely important to Renton. In addition to needing these freeways to serve residents and businesses in our community, we also find our surface streets get seriously congested from cut-through traffic as out-of-town commuters look for ways to avoid these freeways on a daily basis. During peak hours, many of our streets have more pass-through traffic (neither originating or terminating in Renton) than local traffic.
