A three-hour weekend road trip from Seattle to Portland is a thing of the past, says a West Seattle resident who’s been driving there and back for years. As our two metropolises boom and I-5 traffic grows, “We’re all just sitting there.”

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When I ask Bridget Johnson about traffic on her weekend trips to Portland, she sighs.

“It’s just what you have to suffer through to get there,” she said. “You’re just sitting there. You look around at the people in all the other cars, and we’re all just sitting there.”

The native Portlander makes the trip home at least one weekend a month, something she’s been doing since 2011, when she moved to Seattle for graduate school. Now an assistant director at the Burke Museum’s Bill Holm Center, Johnson lives in West Seattle with her fiancé, also a Portland transplant.

Ever since Interstate 5 was completed in the 1960s, Seattle to Portland has been a three-hour drive — at least in theory.

“That’s a thing of the past,” Johnson said, struggling to recall the last time she made the trip there or back in three hours. She says the ride has gotten considerably longer in the past five years.

This is a topic a number of readers have asked me to address: How much longer does a weekend jaunt to Portland take these days?

There’s no easy answer. The state Department of Transportation doesn’t collect data on travel times. But it does measure traffic volume.

Its data show significant increases since 2005 along the I-5 corridor.

Heading south on a Saturday morning, the sharpest rise in traffic is at the Vancouver side of the Interstate Bridge — a 26 percent jump in volume. Northbound on Sunday afternoons, the increases are more modest, as much as 14 percent.

Perhaps that doesn’t seem like such a huge jump — but it’s enough to screw things up.

“It’s like the straw that breaks the camel’s back,” said Mark Hallenbeck, director of the Washington State Transportation Center at the University of Washington.

“There’s a point where you add one more trip to the roadway, and it goes from full but working, to breaking down,” he said. “I-5 has been near the tipping point for a while, and quite often now we’re pushing it over that limit.”

It’s no mystery why traffic is up, of course. Seattle is among the fastest-growing big cities in the country. Portland is no slouch, either, when it comes to growth. As both regions expand, Hallenbeck says, the interchange between the two increases at an ever faster rate.

“Even if we enjoy making fun of Portland,” he said, “everybody likes to go there.”

Both Seattle and Portland have their own internal congestion problems, so the time window for when you can leave and not get stuck in either city’s traffic is shrinking.

And with traffic volumes as high as they are, anything that goes wrong on I-5 in between the two cities — a lane closure, a collision — can have an outsized impact.

Then there are those spots that are always a mess, like Olympia.

“On a Sunday afternoon, you’ve got all the people coming back from Portland and points south, all on I-5,” said Lars Erickson, communications director for the Washington Department of Transportation (WSDOT). “And then you’ve got everyone on 101, coming in from a weekend at the coast or the peninsula. And they’re all converging there in one spot.”

WSDOT has added roadway capacity over the years, but Erickson says there’s no building our way out of congestion. The department’s main focus is squeezing the most out of the pavement we’ve already got — using technology such as ramp meters and driver-information systems, and encouraging motorists to drive at off-peak hours.

And these days, Erickson is also managing expectations.

“You have a lot more people living here and more things happening,” he said. “It’s not like everyone is at home except you on the weekend. You just have to expect traffic.”