Tolls on Highway 520 are making travel faster on that bridge but delaying drivers on nearby I-5 through Seattle and on I-90, new data show.

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In the two months since tolls were imposed on the Highway 520 floating bridge, rush-hour traffic has become much worse for people traveling Interstate 5 through Seattle.

A late-afternoon drive on I-5 from Highway 520 to Interstate 90, which used to average 10 minutes, now takes 18 minutes, according to the INRIX traffic-data firm.

On the nearby I-90 bridge, traffic has slowed, too, by an average 10 miles per hour.

Tolls on the Highway 520 floating bridge have been a boon to people willing to pay up to $3.50 at peak hours. So much traffic has departed that drivers can go 20 to 30 mph faster and cross Lake Washington in half the time.

When tolling began Dec. 29, cynics observed that the state Department of Transportation (DOT) had just created a highway for the privileged few.

But there are also signs that massive diversion — around 40 percent at the start of the year — could be easing.

Some drivers, after taking the extra time to use toll-free I-90, were apparently returning to the 520 bridge during February, says INRIX, which shared preliminary data with The Seattle Times.

The bottom line: It’s not clear yet what the long-term traffic patterns will be, company officials say.

“It’s going to take some time to settle,” said Ted Trepanier, executive director of public-sector business for INRIX, a Kirkland-based firm that sells traffic information and driver services to automakers, delivery fleets and the public. It collects data from satellite-enabled devices in vehicles and smartphones.

The state DOT’s tolling director, Craig Stone, has said it might take half a year for traffic in 520’s sphere of influence to reach equilibrium.

Jim Bak, an INRIX spokesman, wonders if congestion and diversion will forever fluctuate.

“We haven’t seen Mariners baseball season yet. If the team performs and people start filling the seats, that’s going to make for some interesting commutes,” he said.

Just the beginning

State officials warn that tolls are still in an early stage.

January was a low-traffic month because of the New Year’s and Martin Luther King Jr. holidays and an ice storm. So February was really the first test, said toll spokeswoman Patricia Michaud — and Seattle Public Schools took off Presidents Day week, easing traffic pressure.

As for the view that the DOT is creating a costly executive highway, Michaud said transit users are benefiting, too, from much faster trips.

Ridership on cross-520 King County Metro and Sound Transit lines has grown about 10 percent, to an average 27,700 daily boardings, Metro spokeswoman Linda Thielke said. More buses cross the lake with standing-room only.

Another change is more people crossing I-90 in off-peak hours, which eases the strain at rush hour. And traffic engineers have spotted a little surge on 520 just before 7 a.m., when drivers pay $2.80 to cross instead of $3.50 at the top of the hour, Michaud said.

Drivers have been more likely to avoid 520 in off-peak times than in commute hours, a positive trend for DOT revenues.

Overall cross-lake traffic is down 6 percent, a mid-February DOT weekly update said, a finding that will be a bright spot for environmentalists, and the federal transportation department, which has helped subsidize transit and toll equipment as an experiment in traffic relief.

Highway 522 around the lake’s north end was 12 mph slower than normal in February, a sign that drivers are trying out that route as an alternative, INRIX said.

The money

Besides the good or ill effects on traffic, the 520-bridge volumes are crucial to pay for a new bridge — and for what they say about future toll roads including the Highway 99 tunnel, bypassing downtown Seattle at the end of 2015.

The DOT hasn’t reported income from the 520 toll yet but anticipates about $1 million a week to start.

Washington state sold $519 million in toll-backed bonds last fall, the first group of a possible $1.7 billion in bonds supported by 520 tolls. The money would help pay for a new $4.65 billion, six-lane land and lake crossing from Bellevue to Seattle.

Michaud said the numbers to date — a drop of 30 to 40 percent in total traffic — are in the range the DOT expected, so they bode well for the early financing. A study last year predicted traffic would initially drop 48 percent and gradually return. to normal.

But the spinoff congestion shows thousands of drivers think a $3.50 toll is too high. Is the bridge income worth clogging other roads? Should the tolls be reduced? Should the state toll more highways but at a lower rate?

Any rate tweaks would require more than just two months’ experience and would require action by the Transportation Commission, Michaud said.

Meanwhile, proposed budget language in the Legislature moves closer to tolling I-90, by telling DOT to conduct an “environmental review” and to take public comment — both requirements to eventually enact tolls on that route.

Gov. Chris Gregoire and state Treasurer Jim McIntire have said I-90 tolls are needed to close a $2 billion finance gap on Highway 520, and lawmakers have chatted about the idea for years. A backlash would be likely, and initiative promoter Tim Eyman has spoken out against I-90 tolls in the past.

Pro-toll legislators often have cited the 520 diversion problem — rather than merely the need to raise money — as a reason to toll I-90.

So far this year, I-90 has enough room to soak up newly diverted cars, but that bridge is nearing its “sweet spot,” where much more traffic would cause a speed and capacity breakdown, said INRIX’s Trepanier.

“From a traffic-engineering perspective it makes a lot of sense to toll both,” said Trepanier. “You get a lot of efficiency so you get balance. You don’t just want to put it on the backs of people who drive 520. You spread the pain evenly.”

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