Earlier this month, Ray Dubicki laid out why the Ballard Interbay Regional Transportation Study (BIRT) and its accompanying Ballard Bridge Replacement Study are failures. If you missed it, I recommend giving it a full read, but in short: the new plans are the same as the old plans, outdated and ill-equipped for the challenges that are coming to an area incredibly diverse in land use, with light rail on the way.

Not only is the siloed approach to multimodal planning completely faulty, but the actual data that guides the conclusions reached in the studies is bunk, a relic of an era that Seattle should have left behind long ago–one that assumes more car traffic in the future on city streets as a given.

We saw this coming with the release of the Magnolia Bridge Study last year. In that study, the Magnolia Bridge itself was only forecasted to see modest increases in vehicle traffic by 2035, but every single nearby project that could impact traffic was combined together to spell out an increase of 33% in the number of vehicles using 15th Avenue W at Garfield Street by 2035. That had the tidy effect of making the in-kind replacement of the Magnolia bridge perform better than all of the other studied options, which all depended on traffic utilizing some stretch of 15th Avenue.

Projected traffic volumes from the Magnolia Bridge Study. (City of Seattle)

A 33% increase in evening peak hour travel, including a 45% increase in northbound traffic headed to Ballard, is not rational. Therefore all of the outcomes from the study on the replacement for the Magnolia Bridge are sitting on shaky ground from the outset. And now this year, we have the counterpart study for the Ballard Bridge’s replacement.

There, the traffic volumes being used to justify all of the alternatives are on even shakier ground than the ones further south for the Magnolia bridge. Whereas the Magnolia one used units of new housing being developed in Queen Anne, Belltown, and, yes, Ballard to come up with a percentage, the Ballard Bridge study doesn’t even bother to do that.

No, instead it assumes that the current Ballard Bridge will completely fill to capacity, and works backward from there. From the study:

Year 2040 traffic volumes for the No Build (No Improvement) condition were estimated based on the capacity of the Ballard Bridge, which is estimated to be 2,800 vehicles per direction at the posted travel speed. Future volumes could exceed the capacity with crunch flow at a volume-to-capacity (v/c) ratio of about 1.2, which is a peak-direction volume of about 3,360 per hour. For the PM peak hour, the growth in northbound traffic from the existing 2,620 vehicles per hour to 3,360 vehicles per hour would reflect a compound growth rate of 1.2% per year, higher than the historic growth rate described previously, which has been less than 0.1% per year. Reverse direction traffic was also assumed to increase by 1.2% per year, which would reflect a traffic volume of 2,580 in the PM peak hour’s southbound direction.

In other words, the Ballard Bridge study assumes that by 2040, 5,940 vehicles per hour during the evening rush hour will use the bridge. Yet data going back thirty years shows that the Ballard Bridge has never carried that many cars–not even getting close to that number when the Fremont Bridge was under construction and traffic was diverting from the east. It assumes traffic volumes will grow twenty times faster than they have been growing.

We can add that forecast onto the historical data showing afternoon peak hour volumes to see how absurd it is:

PM Peak Hour Volumes on the Ballard Bridge, from the Ballard Bridge Replacement Study, with 2040 volumes added by the author.

Projecting growth like this for the Ballard Bridge goes against all available evidence. Between 1995 and 2019, the Ballard urban village added 5,665 housing units, an increase of 93%, to say nothing of the houses added in Greenwood-Phinney Ridge, Crown Hill, Fremont and other urban villages in North Seattle where residents would be likely to use the Ballard bridge. The amount of afternoon traffic on the Ballard bridge has gone up less than one tenth of one percent every year, 0.06%. At NW Market Street and 15th Avenue NW, the vehicle-centered heart of Ballard, traffic volumes were nearly identical in 1998 and in 2019.

In Magnolia, the unrealistic traffic forecasts merely serve to prop up the option of replacing the existing bridge. In Ballard, the impact is worse, with the Seattle Department of Transportation (SDOT) being pushed to construct new highway style infrastructure with the Ballard Bridge’s replacement. The three options moving forward for that replacement all include a design for a Modified Single Point Urban Interchange (SPUI) on the south end of the bridge at Emerson Street. This cloverleaf-style interchange, shown below, would ensure that the maximum number of vehicles can access the bridge, essentially ensuring that the traffic volumes that have no historic precedent will come to pass.

The modified Single Point Urban Interchange (SPUI) option that is being considered with all Ballard Bridge Replacement options. (City of Seattle)

How was the Modified SPUI arrived at as the only acceptable option? Because of those same traffic forecasts. A normal signalized intersection, a regular traffic light, was considered but “even with dual left-turn lanes and right turn lanes on all approaches and multiple through lanes, the intersection would operate at a very poor level of service (average vehicles delay in excess of 160 seconds, and a northbound queue that could exceed 1,700 feet on average).”

This interchange wouldn’t be very far from Sound Transit’s proposed Interbay station, impacting the walkshed and making station access harder, all because of a traffic forecast that assumes a shift to transit usage and telework won’t continue.

The lack of sensible traffic volumes in these bridge replacement studies shouldn’t go unnoticed, and we need to ensure that decisions that could impact Seattle residents for generations aren’t based on them.

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Ryan Packer lives in the Summit Slope neighborhood of Capitol Hill and has been writing for the blog since 2015. They report on multimodal transportation issues, #VisionZero, preservation, and local politics. They believe in using Seattle's history to help attain the vibrant, diverse city that we all wish to inhabit. In December 2020, Ryan started a three-month stint as editor of Seattle Bike Blog.

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Who’s going to sue? Bogus, falsified traffic numbers are grounds for invalidating an environmental impact statement under NEPA, and likewise for invalidating any other type of study required by the government, but someone has to sue to kill it.

Joe Z

“One of the problems for planners is the residents themselves prioritize car traffic whenever asked. For example, both Magnolia and West Seattle made no loss of car capacity the first priority for any new or repaired bridge.”

That’s not entirely true. People in West Seattle have been complaining excessively about traffic, noise, and speeding in their neighborhoods as traffic has diverted from the West Seattle Bridge. There are also constant complaints in city surveys about a lack of parking, which is another consequence of too many people driving. And surveys show Seattleites are very concerned about climate change.

So it seems people want to be able to drive places but they also do not want the negative externalities associated with driving. And that’s just speaking for people who can afford to own a car. And I didn’t even mention that there also isn’t enough space in cities for everyone to drive.

So what we really need is people in city leadership positions to reasonably partition public space between the competing needs of different road/sidewalk users. And virtually all of them claim to care about pedestrians and transit. But the lack of leadership to actually back those statements up with policy is lacking, especially within SDOT. Kudos to Ryan for calling out these garbage traffic studies.

Transit Rider

Am I reading the proposed interchange right? It looks like the D Line stops are going be moved and require buses to exit 15th, mix with westbound traffic on Emerson, wait for a light, loop around, and then merge back onto 15th. Aside from enabling routes 31/32 to share the stops I see no improvement for the D Line itself here. It will only slow it down.

Additionally, the preferred location for the Interbay Link station (along Thorndyke north of Bertona) is roughly halfway between Emerson and Dravus. If this is not changed bus transfers and pedestrian access will be inconvenient at best.

This will also be the result of Ballard Link’s preferred alignment on 14th Ave north of the ship canal. It may well be cheaper and easier but it will likely be too far from the population center to be as effective as it could be.

I suppose these kinds of poorly planned solutions are to be expected from the aforementioned siloed approach as well as our constant undervaluing of people-oriented infrastructure. Higher expectations can make projects more difficult and expensive but they also make them more successful and better investments.

Daniel Thompson

I agree that much of our current planning is based on assumptions of very large future population increases. The PSRC Vision 2050 statement and the Cascadia Report are two examples, even though King Co. population has increased less than 2% every year over the last 10 years, and has been slowing the last few years. These population estimates are being used to drive upzoning, transit, bridge capacity, TOD, and other policy decisions, many of which are very expensive. https://worldpopulationreview.com/us-counties/wa/king-county-population. If the future population estimates turn out correct, it would be unwise to expect future car traffic to not increase in proportion, and to plan for that.

The assumptions for a future Ballard Bridge include both large increases in Seattle’s population, and car traffic to and from Ballard. The author disagrees with the estimates for future car traffic, but accepts large future population estimates to support Urbanist ideals such as upzoning, transit, TOD and so on.

Generally it is better to err on the side of building a bridge that is too big than have population gains make a new bridge inadequate shortly after being built, unless of course an underlying desire is to prohibit car traffic for some other reason. I think the uncertainty of future population growth, and car traffic growth, is driving the Ballard Bridge design, and so the city figures safer to build bigger, if more expensive.

In just about every city in America as population grows so does car use, so if Seattle and King Co. are going to experience large population growth better build a bigger bridge because history shows residents will still favor driving. After all, would we as a region be spending $74 billion on light rail if we did not assume both greater future population and car traffic?

One of the problems for planners is the residents themselves prioritize car traffic whenever asked. For example, both Magnolia and West Seattle made no loss of car capacity the first priority for any new or repaired bridge. Maybe if Ballard or West Seattle had rail the residents would not be so car centric, and so worried about reduced car capacity (or they will even get rail), and the city would be less inclined to build bridges for future population and car capacity growth.

But it would be a real risk to build a bridge that will or might be inadequate for car traffic and to find out that in the future citizens don’t want to take the train, they still want to drive. I suppose the city could at that point tell the residents tough luck, but that is usually unpopular, and citizens in Ballard and West Seattle are smart enough to be involved in the planning of something as critical as a bridge in the beginning to avoid being told “tough luck”.

I understand building a bridge that turns out to be larger than needed is a loss of money, but that is not how neighborhoods think, especially neighborhoods that are some of the last to get light rail, and are getting materially different light rail stations than they were promised in ST 3 with large cuts to Metro planned. Ballard wants a bridge with all the bells and whistles (and capacity) — including light rail — and so does West Seattle. They think, if everyone switches to rail in the future great, or if population growth is exaggerated great, so you have an over-capacity bridge, which is better than the alternative.

Douglas Trumm

This is Doug, sorry for WordPress name glitch: You seemed to have missed Ryan’s point that the history *already* shows us population growth is not leading to spikes in traffic in the 15th Avenue NW corridor: “Traffic volumes were nearly identical in 1998 and in 2019.” Meanwhile population went up A LOT.

“Ballard urban village added 5,665 housing units, an increase of 93%, to say nothing of the houses added in Greenwood-Phinney Ridge, Crown Hill, Fremont and other urban villages in North Seattle where residents would be likely to use the Ballard bridge. The amount of afternoon traffic on the Ballard bridge has gone up less than one tenth of one percent every year, 0.06%.” What are you proposing will be different in the future?

Annual growth approaching 2% per year is actually very rapid. It’s what allowed Seattle’s population to go from 609k in 2010 to 761k this year. That’s 152,000 in a decade. To put that differently, Seattle added a city larger than Bellevue within its border in 10 years. That’s pretty remarkable, and yet traffic on 15th Avenue NW barely budged. We simply don’t need to add car lanes. But transit cannot keep up with population growth forever if we don’t invest in capacity. The urbanist proposition is that we can have population growth without highway widening. In fact cities should shrink highways to make more room for people, if they’re truly ready to embrace being cities.


I agree, these estimates are ridiculous. If we do see population increases in the area, then transit will improve accordingly. If we don’t, then what we have now is sufficient.

Unfortunately, as Daniel pointed out, these sorts of predictions are often exaggerated. PSRC is full of grand, bold, ridiculous visions of a future Puget Sound. They are nothing new, either. Next thing you know, they will propose building the R. H. Thompson Expressway again (https://historylink.org/File/3114).