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October 20, 2014 at 12:13 PM

Your commute takes much longer, but the reason isn’t clear

If you’re serious about driving from Everett to Seattle in time for work, you need to reach Interstate 5 a full 18 minutes earlier than two years ago.

That’s one finding in the state’s annual Corridor Capacity Report, which measured an astounding 57 percent increase in total hours of delay for the entire I-5 corridor through Everett, Seattle and Federal Way during 2011-13.  Congestion on I-405 worsened 39 percent. Strangely, traffic jams worsened even though total vehicle and miles traveled per person remained flat.

That aggravating news should be no surprise to north-end drivers, who endured Monday morning backups of more than nine miles, following a truck crash and fuel spill near Northgate.

The average morning trip from Everett to Seattle took 50 minutes for a solo driver, up from 40 minutes two years earlier. But the “reliable” travel time, which can be achieved 19 out of 20 days, soared to 80 minutes instead of 62 minutes.  An average transit ride, including stops, lasted 49 minutes, compared with 39 minutes for carpoolers.  Total hours of delay per person increased from 5 hours, 27 minutes in 2011 to 8 hours, 40 minutes in 2013.

“A lot of traffic is hitting the system early on, reducing the speed significantly,” said Sreenath Gangula, lead systems analyst for WSDOT.

For instance, in the south end, average speeds dropped below 51 mph by 5:40 a.m., compared with 6:30 a.m. in 2011 in the Federal Way to Seattle corridor, he said. Likewise, the state has documented a long-term change in afternoon traffic, where the southbound I-5 Ship Canal Bridge congests about 1 p.m., instead of two or three hours later.

So why is congestion worse if overall miles traveled didn’t change?

“I don’t have a great answer to that,” Gangula said. He’s reluctant to single out hotspots, such as local tie-ups related to explosive job growth and Mercer Street construction at South Lake Union. “The root cause is, there’s only so much capacity.”  But in some places the capacity has been reduced — for instance, where the city’s Mercer West project has forced Aurora Avenue North to be narrowed to two lanes, as workers assemble the new Aurora overpass above the new two-way Mercer Street.

More broadly, the state’s population has risen 1.7 percent and statewide congestion 1.5 percent in two years. Another possibility is that people are driving more during crowded commute times and less at other times. One data point in Federal Way shows that trend — vehicle trips increased by 9,500 northbound at I-5 passing South 320th Street, said Gangula.

East of Lake Washington, southbound I-405 morning traffic is 36 percent more congested, slowed in part by lane-construction projects, said Mark Hallenbeck, director of the Washington State Transportation Center at the University of Washington.

Things would be worse if not for high-occupancy lanes, Gangula said.  At Northgate, the two HOV lanes carry 33,900 people each direction during the six hours from 6 a.m. to 9 a.m. and 3 p.m. to 6 p.m., almost as many as the 44,900 each direction in the general lanes.  Some 14 million annual transit riders keep the buses 79 percent full in the I-5 corridor, the state reported.

“Transit plays a major role in at least keeping everything in check,” Gangula said.

In fact, the average vehicle-miles traveled per person dipped slightly, by 0.4 percent statewide. But the population grew 1.7 percent, to 6.9 million people, while jobs and retail sales increased, as the post-recession economy gains momentum.

Here’s one page that summarized I-5 findings:

 

 

 

throughput

 

Comments | Topics: congestion, i-5, Traffic

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