Sound Transit light rail over I-90
Sound Transit

In case you missed it, Mike Lindblom had a great piece in the Seattle times this past weekend about the final engineering plans for East link on I-90:

Engineers have to ensure the bridge will remain buoyant when a pair of 300-ton trains pass each other, and that the high-voltage current that powers the trains won’t stray into the bridge’s pontoons and corrode its steel rebar. They spent $53 million just to design the section across Lake Washington.

The most difficult task is adapting the rails to the movements of the bridge.

Train tracks will cross the hinges and sloping spans between the bridge’s fixed sections and the 1-mile floating deck, like someone walking down the gangway to a boat marina.

Click over to the article for detailed graphics on how it all works, but the gist is that engineers have devised a clever series of bearings to keep the train on the tracks as the bridge bobs, weaves and twists in the wind.

Also noteworthy: the design will allow the trains to cross the bridge at 55mph. Earlier plans had called for slower speeds (35mph) across the bridge. While it probably won’t make a huge difference in travel time, it might have a psychological effect. Seeing a train blow past at 55mph while you’re stuck in traffic could give someone more incentive to switch to transit.

23 Replies to “Sound Transit’s Clever Solution for Crossing I-90”

  1. Earlier plans had called for slower speeds (35mph) …Seeing a train blow past at 55mph while you’re stuck in traffic could give someone more incentive to switch to transit.

    I think the reverse is more likely: Being on a train going 35mph and seeing cars whizz by you at 55mph would make you want to switch back to driving.

    1. Not during rush hour. On the bridge, traffic is somewhere between 20-40 mph for the reverse commute. Once the express lanes go away, it’s going to be like that in both directions during peak rush.

      If you’re trying to get between Seattle and Bellevue between 4.30 and 6pm on a weeknight, Link is always going to be faster. Trust me, I’ve done that drive in both directions many times, on bus and as an SOV.

      If the bridge is clear and cars are going 70, sure they’ll pass the train. But the idea of SOV traffic on the bridge being reliability above 50mph during rush hour is laughable,sorry

      1. 430 to 600 pm. That’s 1.5 hours. What about the other 22.5 hours of the day when cars will be faster?

        If speed is psychological, then people will more likely choose to drive for a vast majority of the day.

      2. GK, I think most drivers are smart enough to realize that the roads are very different during rush hour than they are off-peak, and they will choose the appropriate mode for the appropriate time of day.

      3. The express lanes will be back by the time Link opens. They’re squeezing in two HOV lanes on the outer sides.

      4. GK–The cars could very well be faster at 11 PM, but there won’t be very many cars out there to notice who’s going faster than the train. On the other hand, at 5:30 PM, there will be LOTS of cars whose passengers will notice who’s going fast and who’s not.

      5. @MIke Orr – no, not really. The two new HOV lanes will be open the same time the express lanes close (like in a few weeks), so the total number of lanes on I90 won’t change.

        So that means one less lane going in the peak direction, which I think is going to have a huge impact on congestion during peak. The reverse commuters (especially reverse bus) will benefit, but as more and more people live in Seattle & work on the East side, the difference between primary & reverse commute gets smaller. Basically, they are both going to be very congested during rush hour, just like I5 and 405.

        We shall see

      6. The traditional and reverse commute are the same now. It’s just that one has access to the express lanes and the other doesn’t.

  2. The article talks about halting rail service if sustained winds are 40-50 mph. While this event is described as rare, I would think that the forecast for a possible event would be more common — even a few times a year.

    With this in mind, how would the occasional “bus bridge” work? Will buses be able to stop at the rail platforms? Assuming the answer is no, how will the buses load and unload — at Judkins Park or IDS on the west side, or Mercer Island or South Bellevue on the east side?

    1. The bridge will also close for buses and cars if the winds get much faster than that. It was closed during the last one or two big winds around 2008 if I recall. A 50 mph wind is the same speed as a car on the freeway, so it’s like a car hitting you sideways.

      1. I’ve been hit by 50+ mph wind and survived. Believe me, it’s not the same as getting hit by a 50 mph car (note: I don’t have firsthand experience here). You need to figure mass of the wind versus a car and the resulting momentum. A car is going to go from 50 mph to 0 mph, whereas the wind particles will deflect, lose some speed (and transfer momentum), but ultimately keep going on their merry way.

        Closing the floating bridges is partially due to the increased strain induced on the bridges by sustained winds, not so much the danger to the cars, although waves washing over the bridge is a concern.

    2. They would need to close if winds hit 40+ mph from the north, or 50+ mph from the south. The north wind event is a 10-year event. The south wind event is more common (I don’t know the frequency) but as Mike Orr notes, at 50 mph WSDOT is considering closing the bridge anyway (SR 520 also). They would also need to alter operations if the north winds exceed 30 mph, meaning only one train on the bridge at a time and probably a slower speed; this isn’t too hard at 10+ minute headways (the bridge is just over a mile long, so worst case you have a train holding on one side for about 2 minutes) so I doubt there is much impact unless the event is happening at peak, where headways are 8 minutes or less.

      They could probably make a bus bridge work for the north wind event as it requires a fairly specific weather pattern which should be predictable, and the wind speed doesn’t require the full closure of the bridge, just light rail. They could probably even come close to serving the same stations, although I doubt anyone has considered which station(s) to serve with a bus bridge. Typically you only bridge the closure itself (meaning Judkins Park to Mercer Island), but there are multiple factors here. A major constraint is turning trains. There’s a crossover east of Mercer Island and also pocket tracks west of Judkins Park and south of Stadium. Therefore from a light rail operating standpoint you have to turn trains at Judkins Park or Stadium, and it probably makes sense to continue operating East Link trains to Judkins Park, if only to avoid messing up the north-south line with a turn at Stadium. The passenger experience/connection issue would probably be better addressed via a bus bridge between IDS and Mercer Island. This wouldn’t necessarily be ideal because you’re either a) skipping Judkins Park, or b) serving Judkins Park badly (I-90 ramp configuration only gets you to Rainier; 23rd would be a major deviation if going MI-IDS). Either approach wouldn’t be from ideal, but for a once-in-a-decade event it’s hardly a major concern.

    3. These are sustained winds, not gusts. So the wind would have to be blowing at 40 mph or higher for a full minute to count, and I think those may only get forecasted at most once per year. The last one I remember is that big storm that didn’t happen last October. Gusts at that speed are a lot more common of course–most thunderstorms will have those.

    4. High winds also cause large waves that crash onto the bridge surface. That’s another reason to close the bridge.

    5. They aren’t going to halt train service based on a forecast. The bridge is displaced slowly enough that Sound Transit can wait several minutes, maybe 15- to 30 minutes, for sustained winds before slowing or suspending East Link. You can look up a long interview recently with KIRO Radio where Dave Ross dwelt mainly on this question with Ron Lewis of Sound Transit.

  3. What I took away from this article is both a) this is an amazing technical achievement, and b) this has the capacity to be an incredibly expensive single point of failure. I think we need to look seriously at building a secondary route around the north side of the lake, connecting Northgate to Redmond via Totem Lake, as part of any potential ST4 package.

    1. Ya I feel bad for people who commute to the eastside from SnoCo. The gridlock is so bad today that WSDOT just opened the shoulder peak lane there. So I guess the plan is to just wait it out with a single express toll lane for BRT on 405. Hopefully if there is an ST4, that’s the main method to win over Snohomish, add an extension from Lynwood to Totem Lake or something.

      1. I’m ever hopeful for the Ballard to UW line someday and continuing onto a likely ST4 crossing from Magnuson over to the Kirkland area and tying into a light rail line on the Cross Kirkland Corridor.

        It would be a *very* expensive project, and likely would have to be a tunnel. But as we continue to evolve our tunneling capabilities locally it might be viable. At a minimum a line down 405 from Lynnwood around the north end of the lake connecting which would at least give “get home” capability for link.

    2. RossM, I agree with you about the additional line. But in view of some reports on earth-quake readiness of the length of I-5 past Downtown, and general results of a half century’s deferred maintenance, State and nationwide…well, maybe your line can serve a dual purpose. On same day.


    3. @Ross. “single point of failure”? What the…?

      Setting aside for a moment every transportation artery has multiple points of failure (I-90 avalanche zones, Hood Canal Bridge, I-5 south Toutle & Cowlitz river flooding, busted Skagit bridge on I-5 north, SR-530 @ Oso…), ST3 thought about transit connectivity around BOTH ends of the lake.

      There is BRT spanning Shoreline to Burien via SR 522, I-405, and SR-518. And there are pre-ST4 studies to look at upgrading HCT in those corridors in the future.

      Oh, and there’s that brand spankin’ new 520 bridge.

  4. Did I read somewhere that original idea came from Army pontoon bridges? Because if so, I wonder whether any similar innovations can let us build the transit right of way that regional transit so desperately needs.

    All of our freeways (what an Orwellian description, they’re more like either linear detention cells or miles-long impound lots) are jammed solid for longer time every work day. Often so suddenly that escape, or avoidance, is impossible.

    Only cure is transit-only right of way on every major corridor the length and breadth of the region. Like the pace of the disaster itself- fast. So I wonder what techniques now exist that to swiftly build roadways, surface and elevated, runnable by buses only? Warp speed not necessary. 50 mph is five times as fast as now.

    Since some of the worst goes by Joint Base Lewis McChord, any chance the Army could help with something like this? Whatever the politics that shifted defense highways to commuting, still bad to see Federal shield emblems alongside linear death traps that should be emergency routes.

    If what I’ve read about ‘quake readiness of I-5 through Seattle is true, “next one” wouldn’t have to shake very hard to give us a bigger body count than 9-11-2001. How much of the necessary transitway would the cost of one squadron of jet fighter planes build?

    Earthquake of placard-load detonation more likely than terrorism. But to me, anything an enemy would make it a priority to hit merits defense money to build and protect. Anybody know enough more than I do about what I’m talking about for an answer?

    Mark Dublin

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