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

That should be no surprise to North End drivers who on Monday morning endured backups of more than nine miles, after a truck crash and fuel spill near Northgate.

Increasing delays are documented in the state’s annual Corridor Capacity Report. For I-5 commuters, annual delay per person increased three hours from 2011 to 2013 — from a total 5 hours, 27 minutes, to 8 hours, 40 minutes.

And the “reliable” travel time for that Everett-to-Seattle drive, for solo drivers to be sure of reaching work on time 19 out of 20 days, has increased from 62 minutes to 80 minutes. Average travel time increased, too, from 40 minutes to 50 minutes.

To some extent, traffic jams reflect the state’s economic boom, centered on the nation’s fastest-growing city.

Strangely, traffic got slower even though vehicle miles traveled didn’t change. How is this possible?

“I don’t have a great answer to that,” said Sreenath Gangula, lead systems analyst for the Washington State Department of Transportation. “The root cause is, there’s only so much capacity.”

But he has found that congestion begins before daybreak because drivers are leaving earlier for work.

In the Federal Way-to-Seattle corridor, for instance, average speeds fell below 51 mph as early as 5:40 a.m. last year, whereas cars could stay near the 60-mph speed limit until 6:30 a.m. in 2011, he said. Peak traffic still lasts beyond 9 a.m.

“A lot of traffic is hitting the system early on, reducing the speed significantly,” said Gangula.

Likewise, the state has documented a long-term change in afternoon traffic. The southbound I-5 Ship Canal Bridge congests about 1 p.m., instead of two or three hours later.

Another possibility is that more people drive the busy commute routes while cutting back on other trips, keeping total miles down.

Mark Hallenbeck, director of the Washington State Transportation Center at the University of Washington, likes to cite the “Wile E. Coyote” effect. Traffic runs along just fine — until suddenly it drops off a cliff, as in the cartoon. When a system is stretched as tight as Seattle highways, one crash or stall can stifle highway capacity for miles.

Other theories abound:

• Highway 99 has been reduced from three lanes to two in each direction on Aurora Avenue North, where the Seattle Department of Transportation (DOT) is slowly building a new Aurora overpass above the new, two-way Mercer Street. And in Sodo, only two lanes per direction pass the Highway 99 tunnel construction zone.

• Traffic congestion is a leading indicator that is showing up sooner than other measures of economic growth, said Jim Bak, spokesman for the Kirkland-based INRIX traffic information company. Slowdowns for all Seattle-area driving have increased 6 percent in the past year, he said. “This is not about anything else. This is about the economy,” he said.

• Tolls on the Highway 520 bridge have mixed effects. Traffic there is faster while traffic on untolled I-90 is slower, as drivers divert. Drivers going south toward downtown Seattle face less congestion from drivers weaving onto Highway 520 at the Ship Canal Bridge or weaving from 520 to Mercer Street. But a mile south, there’s more weaving from downtown to I-90, making the right lanes slower, said Hallenbeck.

• Lack of roadway. “The major freeway projects in the area are not adding any new general capacity but cost billions,” said Bob Pishue, transportation analyst for the Washington Policy Center, citing the Highway 99 tunnel, Highway 520 replacement and future light rail on the I-90 bridge.

• Adding roadway. Temporary lane shifts or losses on I-405 near Kirkland, where future high-occupancy or toll (HOT) lanes are being built, are one cause for delays that have worsened by one-third from Lynnwood to Bellevue, Hallenbeck said.

• Deteriorating highways. A passing truck pulled up a loose expansion joint on May 29, closing I-5 from Sodo back through North Seattle. Steve Mullin, president of the business group Washington Roundtable, cited that incident to argue that lawmakers in 2015 should pass a multibillion-dollar transportation package — including $1.25 billion to $3.4 billion for maintenance.

Things would be worse if not for high-occupancy lanes, Gangula said.

At Northgate, the two HOV lanes carry 33,900 people per direction during the six hours from 6 to 9 a.m. and 3 to 6 p.m., almost as many as the 44,900 in all the general lanes combined. 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.

Statewide, population increased 1.7 percent from 2011-13, to 6.89 million people. Jobs increased. Yet vehicle miles traveled (VMT) per person dropped 1.2 percent and overall miles traveled increased 0.4 percent, the report says.

State Transportation Secretary Lynn Peterson sees good news in those numbers. They show that people are opting to make shorter trips, or they spend more time in compact, walkable neighborhoods, she said.

“The fact that VMT per capita is stabilizing, it is a success,” she said.

But she emphasizes: “There are more people moving here every day. Because of that, demand for trips isn’t going down.” The long-range future is hard to predict, she says, because of emerging technologies such as self-driving cars.

What has disappeared is a perpetual stream of rising gas-tax money to build more highways. In their latest report, the state’s economic forecasters predict total miles driven in the state will grow less than 0.5 percent a year through 2020, then slowly wane. That is a cultural change; miles driven increased 4.5 percent a year from 1966-90.

Clark-Williams Derry, deputy director for Sightline Institute, points to another reason for what he calls “peak driving.” The roads have run out of room.

“And then there’s the fact that congested roads actually carry fewer cars per hour — so congestion may be in a very small way contributing to the decline in VMT.”

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com. On Twitter @mikelindblom