WSDOT has been granted a noise variance to quicken the pace of construction and reduce traffic impacts for a new reversible transit and carpool connection from the State Route 520 Bridge to Interstate 5 Express lanes.

The state transportation department plans to have the new connector ramp and high-occupancy vehicle lane open to buses in 2023, when the Montlake phase of the SR-520 BridgeReplacement and HOV Program is slated for completion. It would then open to carpools with three or more occupants when the program concludes nine years from now.

Traffic will be accommodated westbound from SR-520 to the southbound I-5 Express lanes during morning peak hours, and then northbound on I-5 to eastbound 520 during evening peak hours. The reversible auxiliary lane would be added within the existing I-5 Express Lanes corridor while preserving the existing four general-purpose lanes.

“The new Mercer Street ramp would eliminate the need for northbound express lane traffic from Mercer Street to weave across four lanes of traffic to access SR 520,” according to WSDOT exhibits submitted as part of the application process for the noise variance.

WSDOT requested a three-year nighttime construction noise variance from Seattle’s Department of Construction and Inspections, arguing it will save money and avoid exacerbating existing congestion on 520, I-5 and the Mercer Street corridor. The request is for a 6-decibel increase in nighttime hourly average sound levels, between 10 p.m. and 7 a.m. weekdays, and 10 p.m. to 9 a.m. on weekends and holidays.

The I-5-to-SR-520 connector qualifies as a major public project, according to the SDCI director’s decision, and under city code could exceed noise limits by 15 and 25 decibels between 7 a.m. and 10 p.m. on weekdays and 9 a.m. to 10 p.m. on weekends and holidays.

WSDOT reported in application documents how the project wouldn’t be economically feasible if exceeding nighttime noise limits is not allowed, estimating an increased cost of $2.6 million to $9.4 million if construction were only to occur in the daytime. It would also negatively affect travel times.

“Demolishing existing retaining wall at the westbound SR 520 to northbound I-5 ramp could require between 225-300 daytime shifts with non-peak freeway lane and ramp closures, compared to fewer than 72 shifts if completed at night,” according to the SDCI director’s decision

The total economic cost of daytime-only construction was estimated at $90 million to $280 million.

The noise variance granted to WSDOT includes a number of requirements, such as maintaining regular communication with the community through its project team and an independent noise monitor. WSDOT will need to set up a hotline and to respond to complaints within 60 minutes of their filing.

Regular noise monitoring will occur, and SDCI and the surrounding community will have to be informed of upcoming nighttime work at least 72 hours in advance. A 14-day notice is required before the initial start of nighttime construction, according to the decision.

Terms of the noise variance are subject to change following a one-year review, to be followed by annual reviews until the project’s completion.

WSDOT will not be performing auger shaking, jack hammering or impact pile driving from 10 p.m. to 7 a.m. weekdays and 10 p.m. to 9 a.m. on weekends and holidays.

The transportation department is designing the project, which will go out for bids in 2020. The Montlake Project is being completed through a design-build contract with Graham Contracting, which submitted an award-winning bid of $455.35 million last year.