#99closure prep: Analyzing traffic patterns on the Alaskan Way Viaduct

Measuring traffic on the SR 99 Alaskan Way Viaduct isn’t as simple as you might think. Traffic volumes vary along the structure. For example, more vehicles drive the section south of downtown than the section near the Battery Street Tunnel.

But no matter how you add things up, the end result is the same: thousands of vehicles will be forced to find other routes when the viaduct temporarily closes on April 29. And that will equal congestion and frustration, especially for drivers who don’t plan ahead.
As we collectively prepare for the #99closure, we’re hearing lots of questions from the media and public about the current traffic patterns on State Route 99. More specifically, folks are trying to understand how those patterns will change during the closure.
There’s only so much you can do to predict how travelers will react when one of the two main north-south highways through the city shuts down. The only certainty is that traffic will be a major challenge. Here are a few numbers to help you understand what all of us will be dealing with.
The big picture
Let’s start at the top. According to a 2014 survey by Commute Seattle, approximately 228,000 people commute into downtown each day. Of those commuters, about 45 percent take transit and 31 percent drive alone. The remaining commuters walk, bike, telework or find some other way to get to their job each day. 
Those numbers account for people, but the easiest way to understand the effect of closing the viaduct is to look at vehicles. According to WSDOT’s annual traffic count, the viaduct carries approximately 91,000 vehicles per weekday between the midtown ramps and the stadiums. By comparison, the Aurora Bridge carries about 74,000 vehicles on the average weekday.
Viaduct traffic volumes
Learning from the past
We’ve closed the viaduct enough times to know how it typically affects traffic. The structure shuts down twice a year for routine inspections. But those closures are intentionally scheduled on weekends, outside of weekday commutes. 
A better comparison occurred in 2011, when we closed the viaduct for a week to demolish the structure’s southern mile. Back then approximately 110,000 vehicles used the viaduct each day, a number that has decreased due to a number of factors, including the curvy detour that was built to carry SR 99 traffic until the new tunnel opens. 
Much has changed since then. For one thing, Seattle has grown. According to the Commute Seattle survey, downtown has added 26,000 jobs since 2010. To go along with that, the city added roughly 6,000 new residential units to the downtown area between 2012 and 2014.
Infrastructure has changed too. Back in 2011 the South Atlantic Street Overpass – which was built by the viaduct program and now allows trucks and other traffic to bypass a busy railroad track near the Port of Seattle’s busiest freight terminal – didn’t exist. We’ve also invested in changeable message signs throughout the city that alert drivers to traffic conditions, allowing them to make choices and more easily avoid congestion.
Getting ready
There are a number of things you can do to prepare for the upcoming closure. Our dedicated web page, www.99closure.org, is the best place to start. It contains a wealth of tips, maps and other resources designed to help you modify your commute.
We’re getting ready too. WSDOT and its partner agencies have developed an extensive plan to help keep people moving during the closure. Efforts include, but are not limited to: 
  • Up to 22 additional King County Metro buses on standby to help maintain transit schedules. 
  • Installation of traffic cameras and license plate readers to more accurately measure travel times, along with electronic message signs to report travel times.
  • Additional WSDOT Incident Response Teams on I-5 during peak commute hours to assist with accidents and vehicle breakdowns. 
  • Signal timing adjustments to handle the anticipated changes in local traffic patterns. 
  • Uniformed police officers conducting manual traffic control in key intersections as needed. 
We put together a detailed list (PDF) of mitigation efforts, WSDOT funding for mitigation, and other highlights of the traffic control plan. 
Pack patience
While there’s much you can do to help reduce congestion, there’s no getting around the fact that this closure will challenge even the most patient commuter. Even a quick glance at the numbers proves that closing a major highway is never easy, no matter how much you plan ahead.
Hopefully it helps to remember that this is vital work. Replacing the viaduct is an important safety priority, and we’re looking forward to completing a project that will transform the city and the SR 99 corridor. In the meantime, thanks for your patience. Please know that we’re doing everything we can to keep traffic moving during this closure, and we appreciate you pitching in to do your part.
Alaskan Way Viaduct
The Alaskan Way Viaduct will close for approximately two weeks beginning on April 29, 2016.