Rush Hour: A Map Of Puget Sound’s Worst Traffic

Jul 10, 2014

On southbound I-5 near the Ship Canal Bridge, traffic merges onto the highway at 45th Street on the right, but then must weave left four lanes to transfer to state Route 520 right after the bridge.
On southbound I-5 near the Ship Canal Bridge, traffic merges onto the highway at 45th Street on the right, but then must weave left four lanes to transfer to state Route 520 right after the bridge.
Credit Flickr Photo/Lonnon Foster (CC-BY-NC-ND)

It’s been a bad summer for driving in Seattle.

Several accidents have shown us that surface streets cannot handle the traffic load when Interstate 5 or Highway 99 choke up. Traffic and design issues on major routes have been difficult even without an accident.

The traffic data company INRIX says even before the recent spate of bad luck, Seattle-area drivers were spending 38 hours a year in their cars – four more hours than in 2012.

Click on the highlighted parts of the map to see rush hour and "free flow" commute times:

‘Lots Of Moves’

The worst commute in the Puget Sound region is I-5 from Northgate to Union Street in downtown Seattle. That stretch of highway is at its worst during the afternoon commute.

This route on I-5 presents a special difficulty to the state’s highway department. The Washington State Department of Transportation has made improvements to Interstate 405 in Bellevue to make it a less-congested drive. But I-5 from Northgate is different. Just south of Northeast 65th  Street, three lanes start to spit a large volume of cars onto the highway, one on the left and two on the right.

The two lanes on the right – the Northeast 50th and 45th Street exits – create a choke-point because some of the merging cars are headed for the State Route 520, accessible by a left-sided exit just past the Ship Canal Bridge. Those drivers – most of them headed for the Eastside – must cut across four lanes of traffic and the result is a slowdown felt clear past Northgate, four miles away.

This move has a name: a C-class weave. As cars enter the congested highway, drivers already on the road start to change lanes, trying to get away from the hoard of new arrivals.

Credit KUOW/Kara McDermott

“Lots of moves,” said Mark Hallenbeck, the director of the State Transportation Center at the University of Washington, who ventured out into the heart of Seattle’s traffic congestion on a recent afternoon to show what is going so wrong.

“See all this movement?" he said. "One of the problems in congestion is more and more people try to make these moves which actually limits how quickly the road will recover.”

Of the cars entering I-5, one Toyota Camry begins its weave to the 520, disrupting traffic flow on every lane.

“That’s his third lane change," Hallenbeck said, pointing to the weaving Camry. "See how we’re starting to pick up here? We’re through the worst of that weave. There goes the Camry, that’s his fourth. There’s our first official C-class weave person, who made all four movements.”

The Camry sped up over the Ship Canal Bridge and made for the exit, leaving disorder in its wake. It took about 10 seconds for flow on the highway to recover, a delay transmitted and magnified as it moved up the highway.

I-5 Ship Canal Bridge under construction in January 1960.
I-5 Ship Canal Bridge under construction in January 1960.
Credit Flickr Photo/Seattle Municipal Archives (CC-BY-NC-ND)

No Design Workaround

The designers of I-5 in the 1950s did not foresee the wrinkle with their plan.

When they designed this section of I-5, they didn't know there would a 520 bridge. In the decades before Microsoft colonized the Eastside, it was not certain that I-405 would be anything other than a Seattle bypass route. Interstate 90 was already going to cross Lake Washington, so it was thought that there would be no need for a second bridge. Therefore, designers did not consider how to join I-5 to the 520 bridge until much later. 

The C-class weave is not the only problem on our highways, which deliver epic commutes even without major accidents. I-405 at Bellevue has improved, but remains a challenging drive. The S-curves in Renton are infamous, and I-5 north into the downtown is so challenging, the highway department has invested in variable speed signs to warn drivers of slowdowns. 

All this is a headache for drivers, but it also has an economic cost. Boeing has complained that congestion delays key deliveries. Microsoft invested in commuter vans so traveling to work doesn't stress their blue badges – their full time employees. Amazon moved downtown from Beacon Hill so everyone could use public transit.

That’s fine for now. But Hallenbeck said if it gets worse all bets are off. “When companies who are mobile choose to go elsewhere then you have a real, real problem,” he said. “You can see if we don’t start to address things where that will happen.” 

The state could throw money at the problem, but Washington's Legislature is at an impasse about transportation funding package. The lack of a package has held up projects, such as the widening of I-5 northbound to the Seneca Street exit.

But ironing out the C-class weave at the ship canal isn’t on the table. Mark Leth, the chief traffic engineer for the region, said in an interview that the cost of the project is too high. He said a design workaround – like an overpass to take people to 520 – could cost more than half a billion dollars.

A row of Google self-driving cars are shown outside the Computer History Museum in Mountain View, California, this May.
A row of Google self-driving cars are shown outside the Computer History Museum in Mountain View, California, this May.
Credit AP Photo/Eric Risberg

Magic Highway

The return of an old idea may eventually help this stretch of highway: driverless cars. These cars – like the ones Google is developing – could travel close together because they eliminate the little human errors that force us to keep big spaces between our cars. It's like an instant bigger highway. The idea of the driverless car dates back to at least 1958, where it was imagined in the Disney film “Magic Highway.”

“It is the next great technological leap,” said Lars Christian, who has written scholarly papers and a blog about the rise of the driverless car. “We should give them a lane as soon as we can. We'll find they have dramatic effect on congestion even in the next five to ten years."

Christian said introducing driverless cars would be disruptive and messy, but would eventually be a good thing.

Leth believes there may be room on the region’s highways for driverless cars. The highway department is looking for a solution for the Ship Canal mess, and that solution does involve a dedicated lane. It’s not clear yet what kinds of cars will be able to use it.

“That is something that will be explored,” he said. “We’re not there yet.”