Graphic by the Author

Traffic in Seattle is notoriously terrible, and the most oft-cited causes are strong economic growth and lack of rapid transit. While both of those factors are at play, it’s underappreciated just how the details or our freeway construction actively contribute to our daily traffic headaches.

Put aside for a moment the myriad complaints about I-5 in Seattle  – that there is zero HOV priority between Northgate and Union Street, that the antiquated and unidirectional express lanes still freely permit SOVs, and that such SOVs are forced to exit in our fastest growing neighborhood (South Lake Union), etc. – there are also basic engineering reasons why our daily bottlenecks occur so predictably.

Consider just the area between NE 45th Street and I-90. In that roughly 4-mile stretch there are six overlapping 4-lane merges, four of which are attributable to Mercer Street alone. They are:

1. NE 45th Street to eastbound SR 520 (4 lanes in 0.8 miles)

Cars headed to the Eastside from NE 45th Street have just the 0.8 miles of the Ship Canal bridge to merge across 4 lanes to access the left-side exit to eastbound 520.

2. Eastbound University Street to eastbound SR 520 (4 lanes in 2 miles)

Cars using the University Street on-ramp are dumped into the left lane of northbound I-5 just as cars are merging left to access the Mercer Street off-ramp. Those headed for SR 520 have a bit longer (2 miles) to cut across all lanes of traffic to access the SR 520 on-ramp.

3. Eastbound Mercer to Eastbound SR 520 (4 lanes in 0.8 miles)

4. Westbound SR 520 to Westbound Mercer (4 lanes in 0.6 miles)

The Mercer-to-520 merges are the tightest in the city, 4 lanes in just 0.6-0.8 miles. Both merges dump you into the left lanes before requiring a quick merge to access right side on ramps.

If you wondered why Metro’s proposed Route 311 from Woodinville to South Lake Union would have used the express lanes via 42nd street instead of merging directly to Mercer, this is why. The merge is too short to be done safely by buses, and frankly cars have no business doing it either.

5. Eastbound Mercer to Eastbound I-90

Cars using the Mercer on-ramp to southbound I-5 are dumped into the left lane just as it again becomes an HOV lane for the first time since Northgateforcing all SOVs to immediately merge. Those headed from Mercer to I-90 have to merge further, needing to cross 4 lanes in 1 mile in to access the collector-distributor lanes where the I-90 ramps are located.

6. Westbound I-90 to Westbound Mercer (7 lanes in 2 miles)

Requiring 7 lanes in 2 miles, this is perhaps the most chaotic of them all. At the I-5 on-ramp from westbound I-90  (near Dearborn), cars merge into the right lane of the 4-lane “collector-distributor” lanes, and they have until Madison Street (1 mile) to merge all the way left to the through lanes. After Madison, the collector-distributor lanes rejoin the mainline in the center-right lane, requiring cars headed for Mercer to make another 3-lane merge in 1 mile to access the left exit at Mercer.


Enough wonk. The gist is that our 60s-era highway engineers cut corners and didn’t anticipate today’s traffic levels, requiring most Downtown-to-Eastside traffic to merge across all lanes of traffic.  Cars using these pathways clog the highway with merging pressure, and transit can’t use these routes for safety reasons, further incentivizing those who need them to drive.

Short of tearing out I-5, we could clean up some of these traffic patterns, but it would require some pretty unpopular tradeoffs. We could restrict SR 520 access from Downtown Seattle to those using the right-side on ramps to northbound I-5 (Cherry and Olive) and prohibiting 520 access from the left-side on ramps (University and Mercer). Heading toward I-90 from Downtown, we could likewise restrict access from left-side on ramps (Mercer) and require such cars to use right-side on ramps (Yale, Spring, and James). We could also prohibit movements from NE 45th Street to 520, requiring cars to use Ravenna Boulevard or Montlake. In each of these scenarios, I-5 would flow a bit more smoothly and collisions would likely fall. Transit impacts to would be mixed, with the improved flow helping operations but with unknown traffic redistribution patterns possibly hurting transit too.

So if you ever wonder why I-5 backs up by mid-day 7 days per week (especially in the reverse peak direction), it’s often less about the total volume of cars and more about the merging pressure so many drivers introduce through no fault of their own. So when your traffic-choked bus finally gets onto I-5, you can direct a good portion of your ire not at the cars with their blinkers on, but at the shortsighted engineers who force them to do so. Mercer’s recent rebuild aside, the ghost of the Bay Freeway helps it keep making messes.

115 Replies to “How Mercer and 520 Hurt Seattle Traffic”

  1. I wouldn’t call it “cutting corners.” Freeways were a new thing back then, and there simply was no research or good data, let alone well-researched design rules. The engineers were literally making it up as they went. As time passed and we figured out what did and did not work, we added rules. If you look at 1960s-era freeways of most big cities that still have the original ramps intact, they usually have some pretty bizarre or bad merges. In many cases, these have been rebuilt, but too often in an urban environment the reconstruction cost is too high, so we just leave it.

    The same can be said for many engineering approaches from the 60s. Another great example is pavement and geotechnical design. In the 1950s AASHTO conducted a series of pavement road tests on the future alignment of I-80 to determine how many repetitions of traffic a section of pavement could withstand. An ongoing problem is that the truck weights of today did not exist in the 1950s, so we are (still today) basing pavement design largely on testing done by UPS or FedEx-sized box trucks in the 1950s, with the results extrapolated, with some correction to those results from lessons learned in the decades that followed.

    1. The prototypes exist on the Alaskan Way Viaduct and 15th Ave NW. The viaduct has some very short ramps and 90-degree turns that look dangerous to me and weren’t allowed by the 1960s. Those who remember the 1940s Mercer Island Bridge that was replaced in the 80s, it had a bulge for opening that every week some drunk driver slammed into. It also had reversible lanes with alternating EEWW, EWWW, EEWW, EEEW patterns and no barriers between opposite-direction lanes.

      1. Admittedly, the 1940s prototypes were even worse – I’ve driven on many of them, Alaskan Way here and a number of urban “expressways” and “parkways” throughout Ohio, including a few that had reversible lanes and tight curves. Locals advised me to not use the middle lane out of fear of wrong-way motorists. The 1960s true freeways were a major improvement over the experiments of the late ’40s and early ’50s, but still had MAJOR flaws – like weaving in downtown Seattle and short death spiral ramps in congested areas. The freeways of the 1980s generally contain few major fatal flaws but still had some issues (cloverleafs were still pretty dominant). You would not find a cloverleaf on a new freeway today. In fact, you really won’t find too many new freeways today; our work now is in fixing the traffic mistakes that were built out of ignorance, and in getting the high volume corridors infilled with adequate mass transit. I personally believe that transit will do more to fix the congestion on our freeways than any geometric improvement. People will “create” trips if there is congestion-free lane capacity. (If you build it, they will come.) Better to find more efficient means of moving people. This is why I wish that my low-tuition state university had offered some courses in rail design as opposed to freeway and pavement design.

    2. Freeways weren’t really new in Seattle by then. The West Seattle Freeway* was built in 1941, and the Alaskan Way Viaduct was built in the ’50s. But many problems of freeways only really became apparent after Detroit started designing cars *for* freeways, so that traffic went much faster than it did when most cars felt unsafe at modern freeway speeds.

      *(For those who’ve moved here relatively recently — the West Seattle Freeway has since been renamed the West Seattle Bridge in most references, hoping to slow people down. But technically, from I-5 to the actual bridge, it’s a pre-Interstate freeway.)

  2. Excellent article. I think the effect that freeway design has on traffic congestion is underrated. This is a great explanation of why traffic clogs up around these areas. There is little that can be done for the area, as building new ramps (so that all on-ramps and off-ramps are on the right) would be politically impossible.

    There are probably other areas, though, where improvements could be made. Rather than spending billions building brand new freeways (167-509) we should be building smaller projects, like additional ramps. For example, traffic backs up quite a bit at 145th because there is no cloverleaf there. As a result, it will be difficult for buses to get to the new station. Taking a lane is too politically difficult, while expanding the road is very expensive. So, too, is building a new cloverleaf, but the result would be a street that would continue to be two lanes (one for general purpose and one for buses) yet move much more smoothly. I’m sure there are dozens of projects like this all over the place that would benefit transit immensely, while letting general purpose traffic move a bit more smoothly.

    1. We should never build “cloverleaf” patterns entering or exiting arterial roads! We should never build highway ramps that merge onto or peel off of arterial roads, and we especially should never do that right next to major mass transit! It’s a pedestrian disaster.

      1. How is that any worse than what is proposed? Six lanes for 145th, well away from the freeway. That sounds just peachy for pedestrians.

        Traffic sucks because people have to turn left. Good point yvrlutyens — I really don’t care what you build. The point is, if you eliminate the left turns, you eliminate the backups, and you can take a lane. Yes, it is butt ugly next to the freeway — nothing new there. But at least a couple blocks away from it, it is much nicer.

      2. Super dislike Northgate Way & I5 underpass area. So many uncontrolled pedestrian crossing points with automobiles going freeway and artery speeds.

      3. @Jen Northgate Way between Stone Way and Roosevelt is pretty much a disaster for its whole length.

        Bad traffic design, pedestrian design so bad they have been erecting walls to prevent folks from using more natural cross points, high speed curb cuts that will probably get someone killed in the next few years…

        The whole stretch needs a safety redesign.

    2. Cloverleafs would be a return to the bad old days where a huge amount of land became zombie land for the ramps and space between them. The spaces are too small to do anything with except plant grass in, and they become ugly no-man’s-lands. All so cars never have to stop for a light or another car no matter which direction they’re going.

    3. Cloverleaf interchanges are one of those designs that has been abandoned. It looks elegant and it has a minimum of elevated structure, but it also required that traffic merging off the freeway mix with traffic merging on the freeway. One group speeding up and one group slowing down. The solution to this is the flyover or stack intersection that requires much more structure but has the exiting traffic leave before the entering traffic enters, so there is no awkward mixing of the two.

  3. Won’t we be moving almost all bus service off of I-5? By 2023 the 41 and peak-only routes from North Seattle will truncate at Northgate or Roosevelt, the 51x routes at Lynnwood, and the 59x and 57x at Kent-Des Moines. East link will asorb the SR-520 traffic.

    1. I think many of the 520 routes will wind up staying there because East Link goes south from Bellevue.

      1. Post East Link, I think the new transit pattern will be 520-to-SLU/Queen Anne at peak rather than Downtown, once the new express lane connection is built.

      2. Sure, but will they stay on till 5 or exit at Husky Stadium? Metro’s long-range vision, at least, assumes the latter.

      3. So the buses would exit at Denny? That doesn’t seem ideal, given Denny is always backed up. Mercer would be so much better, if they could figure out how to make it happen, eventually

      4. Link doesn’t go to SLU, so it would be a three-seat ride with two short segments. Metro has already redirected some peak expresses to SLU and/or First Hill as an experiment, and if they’re successful that will probably become more common. New York has subways and commuter trains but it also has express buses that go to other niches in the network or are preferred by some. As SLU becomes part of downtown if will become more important for regular routes to go all the way through between Mercer Street and Jackson Street and for express routes to consider going to SLU.

      5. The buses will pull off at the 520 station for Link. Then they will use the bus-only turnaround lanes at Roanoke. This was really good timing — the 520 bridge getting replaced right as Link was designed. Otherwise who knows what would have happened.

    2. Yeah the big issue is the bus traffic going across 520 (with those nice new HOV lanes), and the fact they cannot exit at Mercer to serve SLU

      1. Actually, the long range plan for SR 520 rest of the west (funded by Move Washington in 2015) will provide a reversible direct connection from SR 520 HOV lanes to I-5 reversibles, and presumably allow the reversible exit to Mercer for those buses heading to SLU. Should help the peak direction but not reverse commute of course.

    3. 520 bus riders on 520 directly and south of it will use East Link but for anyone coming from north of SR520 using East Link will be slower. Right now from Totem Lake if you ride the 532/5 to downtown and transfer to the 550 it’s 20 to 30 minutes slower to Westlake. Link will be faster than the 550 but not that much faster I think.

      As to dumping everyone on 520 into Link @ UW… Currently it’s barely time competitive. I timed it last year at numerous times during the morning commute late last year. A 520 bus (even a surface one like 252/257) hits Westlake 13 to 15 minutes after the Montlake Flyer stop. Link takes 8 minutes so you’d have to get off 520, up Montlake, down to the station and wait if necessary, all in less than 5 minutes for it to be better. Reliability would be a problem to without that 2nd bascule bridge for dedicated transit lanes. If the reversible off ramps from SR520 into the express lanes are actually built it will be even less time competitive to transfer. That said I sometimes get off @ MontLake and walk to UW station if 5 and downtown are really bad but it requires paying attention to traffic.

      People on the eastside are ridiculously sensitive to time changes in their commutes. It’s like optimizing it is the great game and amusing to me. When the 405 HOT lanes opened utilization at the Kingsgate PR went way down (it now fills nearly 45 minutes later). I honestly think this is because people can pay 75c to get to Bellevue which is less than the bus by far, but I have no way to prove it obviously.

    4. 59x will truncate at Kent-Des Moines? So that adds another 22 minutes of travel time plus another 5 or 10 minutes to transfer onto a Transit trip from Tacoma to Seattle, right? I suppose that’s one way of dealing with the full parking garage at Tacoma Dome Station in the short term: discourage people from using it by making the transit journey difficult (add a transfer, increase the actual travel time). Prediction: any park and ride lot or garage at Kent Des Moines and Angle Lake will be filled to 100% capacity on day one.

      1. Not if the buses don’t terminate at Tacoma Dome. They could go Lakewood-KDM, Puyallup-KDM, Spanaway-KDM, West Tacoma-KDM, etc. That would be effectively moving the transfer point rather than requiring an additional transfer. Aleks proposed something similar for South King County, where routes would terminate at Rainier Beach Station but be extended at the other end to Kent and Auburn neighborhoods. That way people wouldn’t have to transfer once at Kent Station or Renton TC and again at Rainier Beach; they’d go directly from their neighborood to Raineir Beach. And the routes would overlap between Rainier Beach and Kent Station, and Rainier Beach and Renton TC, to provide 10-minute service where the existing trunk route is.

  4. This analysis (nice work) reminded me of a road junction in a town in Germany I was visiting. A 4-lane East-West bridge crossed a river and fed a 4-lane North-South road on the far side. It was a classic “T” intersection, but along the “top” of the “T” was a large hill.

    Instead of using an onramp configuration with left exits/merges or installing a traffic light, the Germans drilled through the hill to build a tunnel for the East-to-North and North-to-West traffic to exit/enter the bridge. The East-to-South and South-to-West traffic used normal exit designs. The end result was no left-merging, all right-hand exits, and totally free-flowing traffic.

    I still remember this because it was not even major roads or a major city and yet it was engineered for maximum operational efficiency.

  5. When you say that the I-5 engineering “cut corners” it is important to be careful what you wish for. An interchange where all the ramps that go to/from the left lanes merge with the right lanes instead would have had a large land footprint, and the impacts on the surrounding neighborhoods would have been worse. As it is, the I-5/520 interchange is, by freeway standards, about as compact as a freeway interchange can ever get. And I don’t think improving the bottleneck is worth bulldozing half of the Eastlake neighborhood to do it.

    One plan WSDOT has, that will likely get built in the next 5-10 years, is to connect 520 to the I-5 express lanes to downtown. I have always been skeptical about this idea, since half the time, the express lanes are running in the wrong direction, and the plan would inevitably increase congestion by creating an additional merge point for the express lanes. Fortunately, by the time this actually gets built, Link will have hit Northgate, so the route 41 riders won’t have to deal with it.

    I also think long-term, WSDOT should be moving away from unidirectional express lanes, in favor of bi-directional HOV lanes.

    1. I’ve looked but I haven’t been able to find it – are there plans to connect 520 HOV to I5 express? If we assume 520 is the primary bus route from Eastside to downtown, this strikes me as a very good investment in bus infrastructure.

      Having lived in LQA and commuted using the Mercer exist for several months, I’ve wonder if it’s possible to extend the 520 HOV lanes to cross over I5 specifically to allow buses to access the Mercer exit. There might be a bit of property acquisition required, but it looks to me there is enough room to add that. (I think it would be called a flyover ramp?)

      So 520 westbound would cross all of I5 and merge I5 southbound from the right, and then I5 northbound would have a left, HOV only, exit on to 520.

    2. Totally agree. Have told WSDot planners for 20 years (starting with first Trans Laje Study committee of 47 neighbors and electeds) this is bad One directional express lanes terrible. Left on/off ramps even worse.

    3. My guess is that, once the SR 520 West Approach is completed and I-5 in Tacoma is substantially complete (both in 2017, although there’s some Tacoma HOV work on SR 16 through 2020), we’ll hear rumblings of I-5 reconstruction through Seattle.

      The SR 520 connection to the express lanes has been tossed around, but I think WSDOT is more and more realizing that 520/I-5 will need a new design, which will have repercussions that extend both directions on I-5.

      1. Even if you were to rebuild parts of I5 through downtown to correct the ramp issues the fact is the entire road from South Center to near Shoreline needs to be completely repaved. That will cost over a billion dollars last estimate I saw as the panels must be completely removed and the roadbed fixed in many locations.

      2. There’s also the southbound exits downtown where they’re on the right but each one is on a different lane and there’s only one or two lanes that bypass all of them. That was more 1960s thinking that 80% of people would be exiting downtown rather than passing through, and it leads to imbalanced bottlenecks on the lanes leading to certain exits, and bottlenecks on the few through lanes that are available. That should be part of the redesign, and some of it is under the Freeway Park lid so it may be difficult to change.

      3. @Kyle If it cost a billion to repave from Southcenter to Shoreline, it would have been done already. I would probably peg the cost closer to $15 billion+.

      4. >There’s also the southbound exits downtown where they’re on the right but each one is on a different lane and there’s only one or two lanes that bypass all of them. That was more 1960s thinking that 80% of people would be exiting downtown rather than passing through,

        and part of the reason that didn’t happen is that they provided ANY high-speed through lanes at all.

        Really, in retrospect and if Eisenhower’s vision had prevailed I-5 shouldn’t exist between at least 45th and I-90 if not Northgate and I-90, and should have taken 405’s path instead.

    4. It would be possible to get the I-5/520 interchange in the same footprint, with right-hand exits and onramps. Just shift southbound I-5 east 20′ and dig a new tunnel for the south-to-east offramp and build a new bridge for the west-to-south onramp. It only takes money and some long-term lane closures during construction.

    5. @asdf2 — I agree with all of your points. The freeway through the heart of the city was bad enough. Adding ramps going every which direction would have made it worse. That being said, I can’t help but think that in a lot of cases they could have had the same footprint, but better ramps if they had spent a lot more money. Some of this was obviously done this way because it was cheaper.

      I also agree with turning the express lanes into bi-direction transit. Unfortunately, that is likely to be an expensive project.

  6. Great article, Zach. It always seemed like I-5 through downtown Seattle had a preposterous amount of these sorts of threading traffic patterns but I’d never sat down and thought about all of them individually.

    Like you said, this seems pretty unfixable without wreaking havoc on the entire region and surrounding neighborhoods for long periods of time. Are your proposed on ramp/off ramp restrictions even enforceable? Are there examples of similar restrictions around the country? I suppose a tolling camera system like on 520 could keep tabs on that, but that sounds like it would be phenomenally unpopular.

    1. Not sure how feasible the enforcement of restrictions would be (seems not feasible at all, actually), but you could at least cut back on some of the congestion (what percentage, I’m not sure) by indicating with signs that there are only certain ways to get around.

      You’ll often see I-5 signs in the few blocks leading up to an on-ramp from Seattle that point you in a certain direction that’s best for the traffic flow (aka backup) even though you could *technically* go the other way around the block and still get on the freeway.

      So for example, you indicate on signs throughout the city that if you want I-5 to 520, it’s recommended that you go to a certain on-ramp that gives you more room to merge, versus keeping the current system where everything says you’re going to I-5, and hey you’ll figure it out once you’re on the freeway and you have to merge 4 lanes in one mile.

      1. We restrict even more extreme merges, like when you see the “No Access to Olive Way” sign at the University Street on-ramp. Yeah, these would be tough to enforce, but signed nudges still could help.

      2. Not opposed to this if it actually has some impact on congestion, but man, it already takes a master’s degree to comprehend how to get around downtown Seattle in a car. I’ve lived here most of my life and still get confused in the rare times when I have to drive down there. I don’t envy out of town drivers.

    2. There is another possible solution which is simply to close some ramps entirely. I know it has been discussed for the Spring/Seneca ramps both to allow striping another through lane in downtown Seattle and to eliminate some congestion both on I-5 and in downtown Seattle.

      Another thought would be to make all on/off ramps between.the stadium ramps and Mercer HOV only.

  7. How common in practice is the westbound 520 to eastbound 90 (and reverse)? Such a U-shaped freeway drive seems silly. If you’re going to the central district, get off at Montlake from 520 and Rainier from 90. Going to Bellevue or 405? Take 520 if you’re on 520, and 90 if you’re on 90. Mercer Island? Take 405 to 90 if you’re in Bellevue. If you’re coming from Montlake or UW, then it might make some sense to go 520 to 5 to 90, but you could just get on I-5 at 45th and do it that way too.

    1. That’s not particularly common, for the reasons you mentioned. The real issues here are that the most common merges — 520/90 to/from downtown Seattle exits — require the most merging, and they’re the most-traveled routes.

    2. 520 to Mercer is the biggest deal, with Eastsiders going to Seattle Center and the like.

    3. The biggest one I’ve run into with trying to tell other tourists how to get around has to do with getting to Volunteer Park or the Arboretum from Seattle Center. They all have GPS or Google Maps and all of that tells people to take Mercer to I-5 to 520 to 24th, or even worse Mercer to I-5 to Lakeview. I’ve told people that involves changing about 5 lanes in 20 feet and you can’t actually do it that way.

      1. In the bottom right of Google Maps, you can Send Feedback. I have just done so for the route from Seattle Center to the Botanic Gardens, suggesting that drivers should be discouraged from taking routes that require such merges.

    4. I live in Bryant and do this regularly when going to destinations anywhere south of I-90, be it along Rainier Ave, Factoria, or Issaquah and beyond. Taking Montlake to 520 to I-5 is much more efficient than taking NE 45th, 50th, or 65th (*at times of day when SB Montlake is not backed up). Plus I save on not paying 520 bridge tolls. I will take 520 if I see that I-5 or I-90 is slower than 520 and I-405.

  8. All of this merging has a huge knock-on effect for the downtown streets, of course — it’s not just clogging the freeway.

    For example, the Yale/Howell SB I-5 on-ramp, which is very short and feeds directly into the thickest part of all this SB merging, is a nightmare. It joins the freeway right before the other downtown exits and where the freeway splits to push I-90 traffic to the right. Nobody getting on I-5 here will ever be exiting right back into downtown, so you’re faced with a minimum of 2 lane merges just to stay on the freeway, plus another lane if you want to flow onto I-5 and not be in the distributor lanes for I-90.

    The Howell St on-ramp is extremely short, fed by really short (and super congested) blocks all along Howell and of course Denny. On weekdays, Howell (and therefore Olive) is backed up all the way to 5th Ave bumper to bumper because of how backed up that on-ramp gets. This of course also effects the Olive Way traffic trying to get to NB I-5 as well since it’s a shared street.

    The craziest part about this is that it’s not much a Monday-Friday commute time issue. Howell and Olive are backed up several blocks from I-5 roughly 12 hours a day 7 days a week.

  9. You could rip out the express lanes entirely, move the mainlines toward the center of the current footprint and than make the roadway a uniform 4 lanes north and south with an HOV Lane in the Center and there’s your space for making the 520 offramps and Mercer on and off ramps merge onto the same side.

    Cost: Unknown

    1. The express lanes are a quaint relic from when most people commuted from north Seattle to downtown and nobody commuted to the Eastside.

    2. If you asked for those lanes to be HOV+ truck traffic I bet you would have them set up that way within a year. The trucking industry probably hates not being able to get through Seattle in any sort of reasonable fashion.

    3. I’d love to see the express lanes up through Lake City way to simply be a full-time 2-way arterial with 520 access. I-5 can be the way to try to avoid Seattle, the express lane footprint can be the way busses and everyone else tries to avoid I-5 for intracity movement.

      Won’t happen because eventually I-5 through the city will need some sort of big dig– the ship canal bridge and the collanade above eastlake and the bridges and tunnels will all reach the ends of their lives, at which point they’re going to have to try to replace them while also maintaining some lanes of throughput. And they’ll need every foot of right-of-way to do that. To spend 10 years big-digging, taking forever on a megaproject because they need to keep the highway open while building its replacement.

  10. I wonder if improving the Spokane/99 interchange would help the Mercer issue? Currently from the south, the only reasonable way to get to e.g. Interbay is I5->Mercer, because the Spokane viaduct westbound *has no connection* to 99 northbound (that interchange seems to have been built assuming people only go from West Seattle to downtown and vice versa, and are uninterested in any other directions of travel). If that connection were improved to offer more options, it might help distribute traffic flow to make better use of 99, and eventually make use of the multi-billion dollar tunnel we’re building.

    Also (my real motive here) with a re-designed interchange we could push hard for dedicated transit lanes for the buses that will slog through the area for the next several decades until light rail is built.

    1. I drove to/from West Seattle regularly for ~ 5 years and this was always in my mind the biggest highway config thing in the city that never really gets talked about. That interchange needs to be a full interchange. As it’s warehouses and elevated it could be built over the buildings below as it is today. Though I bet that’s grandfathered in and would never be allowed today. :-P The transit benefits of being able to spin off in all 4 cardinal directions from there could be huge if done right.

    2. Totally agree – that is a huge missing link in the system.

      Perhaps the devil is in the details but it seems like a logical addition.

      A ramp from WB Spokane to NB 99 would put that traffic in the 99 bus lane, but if the bus lane started ~500′ further north that shouldn’t have a huge impact on bus service.

      1. Their was a WSDOT project to connect the bus lanes of I-5 to the SoDo busway. I don’t know what happened to it, but it might have stalled as buses got kicked out of the tunnel.

  11. No, also blame the driver of those cars. If people didn’t choose to live miles away from work, or choose to make unnecessary trips (40% of all rush hour traffic is not work-related), there would be no congestion, regardless of road design.

    Also, no mention of I-5 southbound lanes being reduced between the ship canal and convention center?

      1. It is the fault of those unintelligent drivers. As everyone knows, if the entire population lived in Capitol Hill, this simply wouldn’t be a problem.

      2. If drivers would only look at their GPS rather than thinking they know better than it, all these bottlenecks would disappear.

  12. Zach,

    I’m pretty sure that the left accesses at 520 were to preserve more of the city. The City strongly wanted to keep the north-south freeway footprint one block wide. Mercer may be less about footprint than the hillside behind and preserving Eastlake as a through street.

    Your ideas for limiting access from Mercer to the cross-lake bridges is a great one, but it would be howlingly unpopular and pretty hard to enforce. The northbound Mercer

    1. I might be missing something, but why would a right-side freeway interchange take up more land? As it is, just south of the interchange, you’ve got NB-EB exit, NB mainline, express, WB-SB entrance, SB mainline. Why couldn’t you configure things to switch around the WB-SB entrance and SB mainline, and take up the exact same amount of space? Sure, you’d need a different curvature for the WB-SB entrance, but I don’t see why it’d need to eat up any more space.

      1. Makes sense to me.

        The left exit from SB I-5 to EB 520 also seems to be unnecessary. A cursory look at Google Earth suggests the 520 ramp could have perhaps shared the right exit with Boylston and then ducked under the freeway along the lines of the current tunnel alignment. The SB 5 mainline would probably need to be aligned further east but without the left exit for 520 there would be space available.

        It almost makes me wonder if some 1960s engineer really, really liked left exits…

      2. Yeah, I think it was money, more than anything. You folks who are new to town forget that Seattle used to have quite the reputation for being cheap. Why does connecting from I-5 south to SR 520 require getting in the left lane? Because we are cheap. Why do all our professional teams play in an ugly cement mushroom? Because we are cheap. Why did we put the sewage treatment plant in the nicest park in the city? Because we are cheap.

        I kind of miss those days. Being cheap is better than being stupid. We have two state of the art stadiums, with two different teams playing in them, but we lost our longest running franchise. Oops.

      3. You folks who are new to town forget that Seattle used to have quite the reputation for being cheap.

        Unless you were born in Portland, which has a reputation for doing things even more cheaply, for the most part (We did manage to do a deep bore tunnel and put the sewage treatment plant in a spot where most of the people who can smell it live in Vancouver…). We have a habit of looking north blissfully and wondering how Seattle manages to pull some of this stuff off.

        Highway 217 through Beaverton and Tigard? Up until the Great Widening Project a few years ago, the thing was unimproved lanes from the 1960s, truly and completely. I think the lane width was only about 7 feet wide or so, but whatever the width they were far narrower than current highway standards considered safe. Somehow, I-5 through Seattle was given modern lane width decades earlier.

        HOV lanes? We’ve scraped together enough money for 3 miles of them.

        Then, there’s the Seattle Transit Tunnel, compared to the Portland Transit Mall.

        So, cheap is all a matter of perspective.

      4. It might be because of the Boylston/Roanoke southbound on-ramp. Or maybe as Ross says, they were just being cheap. But they did have a goal of keeping the freeway within one bulldozed block’s width through Eastlake.

    2. Restricting Mercer Street from accessing 520 and I-90 means pushing more SOV into downtown and onto Eastlake. I’m not sure that I-5 would improve enough with the restrictions to make up for impact it has. I think this is just shuffling things around but until it’s studied, it’s hard to say.

      I can tell you that I am a driver that sometimes merges from 520 to Mercer St. via I-5 South. The reason? It takes twice as long to get off at Roanoke St and head down Eastlake since Valley Street is always a parking lot at rush hour. So frankly, even though cars supposedly have no business doing this merge according to Zack, it’s still the fastest option which is why people still do it. The options around it are even more congested than Mercer Street west bound and I-5 south bound during rush hour.

      1. Yeah the genesis for this post was that I found myself doing it too because it was the fastest, despite the upstream effects I knew I was causing.

      2. Maybe try getting off at Roanoke St and then getting back on I-5 in the right lane at Boylston? I’ve done that several times, though never at rush hour, so I don’t know how traffic would be then.

      3. Everyone on the Eastside thinks 520-I5-Mercer is the way to get to Seattle Center, and few of them know where Roanoke Street goes. It’s not that drivers have “no business” doing the merge but that the entrance should have been on the right like almost all entrances are.

  13. This is one of those complex and messy traffic issues that has no true easy fix. It’s too bad that the 520/I-5 project won’t help – mostly because there isn’t funding to do it right.

    I’ve even heard that there are unfunded design solutions gathering dust at WSDOT, done by past staff.

  14. I-5 was completely botched up from an engineering point from the start. The biggest case in point, you only have two general purpose lanes moving through the downtown core. Everything else is for an off ramp or other redirect.

    If we were going to build a tunnel for a highway in this town, it should have been here, and not on the waterfront.

    1. That’s not true. There is an unbroken lane (the leftmost one) which takes the right fork at University and then merges back into the main lanes south of Edgar Martinez. I use it EVERY TIME I drive through Seattle (except of course when the express lanes are southbound) because it’s relatively smooth and almost completely freak-out-free. It’s not very often that I drive that way, but it’s really nice to know about that lane.

      1. continued

        I even prefer it to the HOV lane when my wife and I are riding together, because it doesn’t have the azzoles in giant rigs trying to squash me for not going 75.

  15. As someone who occasionally does the westbound I-90 to westbound Mercer route, I don’t see how a 7 lane merge is required for this maneuver, unless you are doing unnecessary lane changes to skip around slow traffic.

    There are two lanes on the ramp between I-90 and the collector/distributor, which both come in as their own lanes. The right lane becomes an exit only for James, so that would require a merge. The left lane is continuous past Madison and only requires one lane change before the collector/distributor recombines with the mainline of I-5 (near the Spring St. overpass). If you go all the way into the left lane of the collector/distributor you are required to merge back to the right again because that lane ends near the Madison exit. The collector/distributor then comes in as it’s own lane when it recombines with the main line. From there you only have to merge across 3 lanes of traffic to enter the left side exit only lane for Mercer.

    So, if you start in the left lane of the I-90 ramp, you only need to merge 4 times, and if you are in the right lane you have to merge 5 times.

    I’m not saying that there isn’t a problem there (there is), just want to make sure the information is accurate.

  16. > Short of tearing up I-5

    I dreamed a dream in time gone by….. when cars were gone and life worth living

    1. Tearing out i-5 from Southcenter to Alderwood (making 405 the new 5) is the ONLY answer…

      We all know that but don’t have the will to do it.

      1. As if those of us on the Eastside want a 14 lane freeway . . we already have our open sewer of traffic to deal with. Keep yours.

      2. What do you replace it with? There are a lot of overlapping trips between Shoreline, 45th, downtown, Georgetown, and Southcenter. Possibly too many for a regular 2-lane or 4-lane arterial. You could build two smaller roads to dowtown that are disconnected between them, but then you come back to trips that cross downtown. It’s hard to find a solution that doesn’t come back to a single highway through all of it, and that’s essentially the same as I-5 even if it’s not called that. Although you could turn it into an expressway or boulevard with intersections, and that would help it integrate into the surrounding neighborhoods, but how far you think “These politicians want to slow down my drive!” will fare politically?

      3. You replace it with nothing. Vancouver has the right idea.

        When Ike figured out they were building freeway THROUGH cities instead of just TO cities, he was horrified. And rightly so.

        Freeways don’t belong in cities.

        That our sprawl and consequent traffic patterns now rely on them is besides the point. We would adjust.

      4. You replace it with nothing…. you just build houses on it? Vancouver doesn’t have a sound and a lake on two sides and few north-south streets in between,m and now a million people south of it and most of a million north of it.

      5. Maybe turn it into a giant bike lane on the bottom express lanes for safe, flat, dry and incredibly quick commute into downtown.

        Bike share would spike to 30% overnight.

        Turn the top into a park, and build like six gazzillion ped bridges, stairs and slides all over it.

        Ban SOVs from Edgar Martinez Drive to the ship-canal.

        Jack bertha’s grave to $30 each way to pay for the necessary 5 new in-city subways.


      1. Bellevue and Kirkland are cities too. So much for your dream of shifting all of I5 traffic to 405, by your logic it should also be gone.

      2. 405 was there before most of the people were, and suburbanites say they like freeways, so let them put 405 in their pipe and smoke it.

  17. While the left entrances are certainly disastrous with today’s traffic levels (and are almost never built on new freeways nowadays), there are some things that the original I-5 designers did a better job on. I am thankful for the “braided ramps” for NE 45th and NE 50th, and NE 70th and LCW.

  18. I don’t think you can blame the engineers for the I5 mess. As Anandakos mentioned the City decided to limit I5 to a one block footprint north-south through the city. I had heard that before, in a conversation with a business owner. That has and continues to affect what can be done.
    I don’t see a solution, but who knows.
    Another nasty merge is the one needed to get from I5 south to the Rainer off-ramp. It’s quite demanding.

    1. Yeah, every time I take that exit I cross my fingers and light some incense to every god I can think of. And I’m an atheist.

  19. When I lived on the Eastside in the 1990s, to avoid the scary southbound merge onto I-5, many drivers would exit at Roanoke then turn left onto Boylston which would take them to the far right southbound I-5 lane — a shortcut used especially by those headed for Seattle Center. Don’t know if this is still possible.

  20. A project to reconfigure I-5 – at least at the SR-520 interchange, if not all the way through downtown – could actually gain significant traction if it were bundled with the proposed downtown I-5 lid. Urbanists would get a freeway buried, suburbs would be able to boast shorter commute times to the city, and urban drivers would get a project that doesn’t get their “war on cars” hackles up…it’s a win for everybody.

    The only question is, can we do such a project without it being the next Boston Big Dig (or given that Bertha has already done that, and the Puget Sound Gateway is soon to follow, can we at least stop living up to our boondogglish reputation)?

    1. I invoked a “big dig” above… WSDOT might be game for a big dig, but they’d want to replace the ship canal bridge and every other seismically outdated thigamabob from UW through the ID, meaning an endless megaproject, at which point, sure, put a cherry on top of the brand new superhighway through Seattle and throw in a lid or better intracity movement…

      1. How can the Ship Canal Bridge ever be replaced without taking it out of service? Eventually it will rust and vibrate enough that it become hazardous, and that carmageddon will arrive. But to suggest doing it before it is necessary beyond any doubt would cause a political firestorm.

      2. P.S. Sure, you can renew the decking. But the structure is a monolith; because of the cantilevers it can’t be replaced a girder at a time.

  21. Well what do you expect when you don’t fund a 60s era freeway design to expand or improve it for today’s needs?

  22. Zach, you may be interested in this 2003 Seattle Time article about the need to essentially rebuild Interstate 5 through most of Seattle. Now 13 years later it appears WSDOT still has no legislative direction on this, but work like this could help lay the foundation for a redesigned freeway that is more smooth for cars, trucks, and transit. And it could come, of course, with a freeway lid in the Center City. :)

  23. I didn’t read all the comment so I apologize if this was already stated.

    Hypothetical here – What if we charged people $$$MONEY$$$ for making these expensive maneuvers during congestion hours.

    Install congestion pricing style database linked cameras at the listed entrances to I-5 and at the junction points, then use DATA to enumerate (and charge) cars enter and merge 4 lanes in 0.8 miles.

    This would discourage costly behavior and be a baby step towards CONGESTION pricing.

    How expensive is it to install and run the cameras anyways? I’m guessing the answer is prohibitively expensive unless you rake in some money from charging everyone.

    1. It’s called the 520 toll.

      No one would be doing that maneuver if there was another way to get back across the lake in a similar amount of time. Blocking off that route would only cause more traffic on the arterial streets… to which then I’m sure you’d come up with an ingenious solution involving taking more money from people.

  24. Horn “doesn’t know” this and “wonders about” that. And he was the chair of the Transportation Committee!?!?!? By the way, is that chair reserved for Mercer Island? Inquiring minds want to know!

  25. The concern I have with the proposal is its impact on the downtown streets affected. If you make every commuter seeking to go from west of I-5 to 520 take streets that enter the freeway on the right, you’re going to clog a lot of overpasses and streets that currently have other users and uses, like not just carrying buses to Capitol and First Hills, but other cars as well. Basically, it’s giving some Seattle streets up to Eastside commuters in order to advantage I-5 north commuters. For those of us who never use either I-5 or 520, it’s a lot to take away for their benefit.

    Kind of like what will happen when all the Snohomish County commuters and Eastside commuters crowd the parts of Link they didn’t pay for.

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