By Ethan Bergerson
It's exciting that we're only a few months away from the opening of the new State Route 99 tunnel under downtown Seattle. The tunnel will dramatically change the way people get around Seattle. There will be no mid-town entrances and exits like the Alaskan Way Viaduct has today. The tunnel's entrances and exits will move farther north near the Space Needle and farther south near the sports stadiums. Will your trip be shorter or longer? Better or worse? The answer depends on your destination.
Traffic patterns will change over time
While we're excited about the opening of the SR 99 tunnel, it's important to remember that this is ultimately a safety project, not a congestion-relief project. The tunnel was one part of the plan to replace the aging Alaskan Way Viaduct, which was vulnerable in case of an earthquake. It is designed to work in combination with a new Alaskan Way surface street which will not be complete until 2021.
Seattle traffic patterns will change at different points in time. There will be a big adjustment when the tunnel first opens and another shift when tolling begins, which could be as soon as summer 2019. Traffic patterns will finally settle when the new Alaskan Way surface street opens.
The tunnel will be a more direct route for drivers who want to get through downtown. On the other hand, the tunnel will not have the same ramps to downtown at Seneca St., Columbia St., and Western Ave. as the current viaduct does. So we expect downtown traffic to shift away from the parts of downtown which had been connected to the current viaduct and shift towards the new tunnel entrances and exits in South Lake Union and SoDo.
Predictions for tunnel usage
After the tunnel opens, we expect it will take a few months for traffic patterns to settle as drivers try out different routes to find what works best for them. While not everyone who uses the viaduct today will use the tunnel, we expect other drivers will find that the tunnel is a quicker option than the route they take today.
Our traffic predictions show that before tolling begins, there will likely be more drivers using the tunnel during peak periods than use the viaduct today. There are about 5,200 vehicles per hour which currently use the northern section of the viaduct at peak times (6 - 9 a.m. and 3 - 6 p.m.). The tunnel could potentially carry as many as 7,000 vehicles per hour based on our traffic forecasts.
More changes when tolling begins
The tunnel will be toll-free when it first opens, with tolling expected to begin as soon as summer 2019. The Washington State Transportation Commission has set toll rates ranging from $1 to $2.25 with a Good To Go! pass depending on time of day.
Tolls will cause traffic patterns to change again. When tolling first begins, we expect the number of vehicles travelling through the tunnel to drop to between 4,000 to 5,000 vehicles per hour during peak periods. Drivers leaving the tunnel might choose to go to I-5, Alaskan Way, or local streets.
We also expect a gradual return to the tunnel as more drivers create Good To Go! accounts and get a better sense of the value of the time they save by using the tunnel. We've seen this pattern on the SR 520 Bridge and other toll roads around the country.
Waterfront improvements will be completed in 2021
Ultimately, traffic patterns won't truly settle until the future Alaskan Way surface street improvements are completed in 2021. The new Alaskan Way is designed to work with the tunnel and give drivers multiple options to get into and through downtown. Overall, the two routes are expected to carry 5,000 to 6,000 vehicles per hour during peak periods and will work together to replace the function of the Alaskan Way Viaduct.
Time will tell
Much like a weather forecast, these traffic projections aren't an absolute certainty but they do give us a glimpse into how traffic patterns may change over time. These projections are our best attempts to predict what people may choose to do based on a model of an extremely complex and constantly changing system.
After the tunnel opens, we will continue to monitor traffic patterns and will give updates about what we see. Ultimately, the new tunnel, in combination with several other major construction projects, means that we can expect several years of change before traffic patterns truly settle down.