The planned Highway 520 bridge design guides commuters in buses and carpools from the bridge's HOV lanes right onto the Interstate 5 express lanes — but only in one direction. Commuters in most directions can still expect bottlenecks.

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The public’s reward for paying gas taxes and tolls toward a new Highway 520 floating bridge — besides not having to worry about the current one sinking — was supposed to be the chance to add quick-moving lanes for transit and carpools.

But despite the presence of two high-occupancy-vehicle lanes on the new six-lane bridge, commuters would still hit bottlenecks where Interstate 5 meets 520. That’s because the state’s plan for bus and carpool lanes is designed for commuters who live on the Eastside and work in downtown Seattle.

So if you live on the west side of Lake Washington and work on the Eastside, your bus ride or carpool to reach the $4.65 billion bridge will be just as slow as today. Same goes if you live on the Eastside and need to take I-5 north to work after you cross the bridge.

That’s because the state’s plan to unclog transit traffic at the interchange rests mainly with a single reversible ramp that serves only a traditional Eastside-to-downtown route. The ramp would take buses and three-person carpools from the bridge’s new HOV lanes and guide them directly into I-5’s express lanes in the morning. In the afternoon, northbound I-5 express-lane transit drivers would take the same ramp back to the 520 HOV lanes.

But that’s it.

If you live in Northgate, Shoreline, Mountlake Terrace or Edmonds, your bus or carpool won’t reach 520 until after you slog through general traffic on I-5 and drive single-file through the existing left-side exit, just like today.

The new ramp also won’t help commuters who live in Belltown, South Lake Union or Queen Anne and head to the Eastside, perhaps to work at Microsoft. They won’t enjoy direct access to the new transit corridor via Mercer Street, because the I-5 express lanes are pointing south at that time.

These limitations have been ignored for years, despite traffic from Seattle residents heading east that matches or exceeds that of Eastsiders coming into Seattle.

State Sen. Ed Murray, D-Seattle, says the 520 project is based on a traditional suburb-to-Seattle commute pattern that is becoming obsolete.

“If you’re talking about transit, for instance, in this area, it’s not just people moving from the suburbs to the central city,” he said. “There’s a whole lot of employment areas people are moving through.”

For instance, he notices that Sound Transit’s morning buses from downtown Seattle to Redmond are packed. Another bus route, from Northgate to Bellevue to Issaquah, makes the 520 crossing, too. Both use I-5 general lanes, because express lanes don’t connect to 520. And even when the new bridge is built, they still will have to navigate clogged general I-5 lanes, then merge across 520 to reach the new transit lanes.

Murray doesn’t think the state has a good feel for where and how trips will be made in the late 2010s, when the bridge is done. Nor does the plan adapt to the fact I-5 is full, he said.

“Instead of designing a bridge that looks at moving people, we just designed a bridge.”

Politicians and the public have largely ignored the freeway junction as they focus on other issues, such as the bridge width, how to collect tolls, the Montlake interchange, or whether to add train tracks. Seattle Mayor Mike McGinn is seeking a redesign to put light-rail trains on the new bridge.

But leaders have a bit more time to think about the regional connections, because the state and Seattle governments agreed to hold more design negotiations until October, and the city is spending $250,000 for a last-ditch study of transit and design issues.

The existing four-lane bridge across Lake Washington, built in 1963, carries 115,000 vehicles per weekday. But it has exceeded its life span and could capsize in an earthquake or severe windstorm.

Northern dead-end

Eight years ago, the state’s engineers proposed a ramp from the I-5 express lanes so that southbound buses and carpools from North Seattle could reach the new 520 easily.

But that ramp and others were scrapped to reduce costs when a gas-tax measure known as Referendum 51 failed in 2002, said project director Julie Meredith.

Her explanation reaches deep into history, because the state has enacted a pair of gas-tax hikes, and enjoyed a mostly prosperous decade after R-51. On the other hand, she points out that the project budget for 520 is already $2 billion short.

Adding that ramp would cost more than $100 million, the DOT says. That’s an amount comparable to a half-mile of light rail, or the planned First Hill Streetcar.

Nobody has suggested adding a transit-carpool ramp in that direction in the past few years, Meredith said.

Doing so would require wider overpasses at north Capitol Hill, and might increase construction time.

Capitol Hill resident Steve Silverberg said he asked about the missing transit link during an open house, and DOT engineers called it a political issue they did not want to touch.

This bothered him, since afternoon westbound traffic across Lake Washington is typically bad, especially when there are concerts or ballgames in Seattle. Drivers in the future bridge HOV lanes would need to merge into the general traffic because, by afternoon, the I-5 express lanes are northbound.

“It’s pretty astonishing that this far into the project, and with all that’s at stake, not even the mayor or City Council has spoken up,” Silverberg said.

McGinn spokesman Aaron Pickus said the mayor is looking for ways to improve transit over the 520 bridge. “We are certainly open for a discussion of ways to improve the flow of traffic on I-5, but are still studying all of the options available,” he said.

The state’s traffic study predicts: “HOVs merging back into the general purpose lanes would cause traffic to slow” afternoons. But Michael Horntvedt, transportation manager, said he doubts the delay would be substantial for transit, because the merge zone at I-5 is only a quarter-mile.

Doug MacDonald, state transportation secretary from 2001-07, doesn’t recall any brainstorming about I-5/520 transit links, an issue he says he delegated to others. But in 2010, he says a northerly connection is crucial.

“I don’t have any idea why that would be cut,” he said.

That link fits MacDonald’s view of a future where regional express buses, vanpools, electric shuttles, taxis and private transit such as the Microsoft Connector fan out in all directions, instead of a fixed-rail model that serves a few of the most densely populated corridors.

The best direction

The state’s proposed HOV junction is satisfactory, because it serves the area’s best transit market, Meredith said.

Last year, about 2,200 Metro and Sound Transit bus riders per weekday traveled to downtown Seattle from the Eastside, while only 1,400 rode east, according to the project’s environmental-impact statement. An additional 1,100 or so Eastsiders went as far as the University of Washington.

“Most of the riders coming across 520 really do desire to go downtown,” Meredith said.

Kevin Desmond, general manager of King County Metro Transit, said he’s focused on the Montlake Interchange and the university, and less familiar with the I-5 junction. Overall, buses will operate “reasonably well,” he said.

A greater mystery are the patterns for carpools and vanpools — especially if congestion, or tolls of perhaps $6 per day prod thousands of people to seek a three-person carpool across 520.

Over the next few years, Metro plans to increase bus service through the 520 corridor, using a federal transit grant. The old bridge is supposed to be tolled in 2011 — likely pushing some commuters to buses — as an experiment in reducing congestion.

The state plan “will work” for the Sound Transit bus services that voters approved in 2008 — two existing routes across Highway 520, plus a new line from Kirkland to the university — said policy and planning director Ric Ilgenfritz.

“Is it optimal? It could always be better, but that’s not a fight we’re picking,” he said.

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