West Seattle Bridge cost-benefit analysis released, and the options are clear (even if the report isn’t)

Today SDOT finally released the long-awaited “cost-benefit analysis” on the various options for repair or replacement of the West Seattle Bridge. It’s 89 pages, it’s dense, and it wins the award for “worst executive summary.” SDOT had to publish a “reader’s guide” today to help people make sense of it. But in the end it tells us what we need to know in order to decide whether we should repair or replace the bridge.

Here’s what it says.


The stated purpose of the cost-benefit analysis is “to evaluate the benefits, drawbacks, and rough-order-of-magnitude (ROM) costs of multiple alternatives to determine whether it would be in the City’s best interest to further rehabilitate the existing bridge or immediately pivot towards a replacement.”  In a nutshell: they took six options, evaluated how they would perform against a number of goals for the bridge, estimated what each would cost, and derived a “value index” for each option to allow, at some level, for a comparison between wildly different alternatives.

Let’s walk through the six options, how their performance ranked, the costs for each and then the ultimate value indices.

The Six Alternatives

The report presents two repair alternatives, three bridge-replacement alternatives, and a final alternative to scrap the bridge entirely in favor of a tunnel under the Duwamish River.

Three of the alternatives can be set aside pretty quickly: Alternatives 1, 3 and 6.

Alternative 1: Shoring

This option adds structural shoring with trusses either underneath or on top of the span. The option to put the trusses underneath the bridge failed because it would encroach on the waterway underneath and potentially impair boat traffic. Putting trusses on top is possible, and the bridge could be reopened in 2024, but it is only a temporary measure for up to five years while a replacement bridge is planned.

Alternative 2: Repairing the bridge with “direct strengthening”

This option adds new steel tendons the length of the span (in addition to the ones currently being added as part of the stabilization work) to increase the strength of the bridge. The latest estimates are that this would restore the bridge to its original expected lifespan: another 40 years. It could be completed and the bridge reopened to traffic in 2022, then around 2062 the bridge would need to be torn down and replaced.

During the examinations of the bridge over the past several months, crews discovered that one of the pier foundations has some issues that are not urgent, but will need to be addressed in the next ten years or so if the current bridge foundations and pier are to extend through their full expected life.

Alternative 3: Partial superstructure replacement

In this option, the damaged middle section of the bridge would be removed and replaced. This option has already been withdrawn from consideration because of feasibility concerns: the engineers were not confident that they would be able to connect the new segment into the bridge in a manner that would ensure its strength

Alternative 4: “Accelerated” superstructure replacement

This option would demolish and replace all of the box girder segments between Piers 15 and 18 (the three center spans of the bridge) and replace them with new box girders segments. The existing foundations and piers would be reused.

The bridge would reopen in 2026, and (except for a tune-up at 50 years) it would be “good as new” with a 75-year estimated lifetime — out to 2101.

Alternative 5: “Accelerated” bridge replacement

This option would replace the entire bridge span, and possibly the piers and foundations. The new bridge would not be box girder, but instead a “cable stayed” form of suspension bridge.

The bridge would open to traffic in 2026, and have a 75-year expected life.

Alternative 6: Off-alignment immersed tube tunnel

This option was added in at the request of advocates in the community. It would build a tube tunnel under the river next to the existing bridge, and after it is open then the existing bridge would be demolished.

It turns out this option is messy, very expensive, and takes a long time to build; it likely wouldn’t open until 2030.


The performance analysis in the report was done by three entities: SDOT, the Technical Advisory Panel (TAP) convened by SDOT to provide bridge engineering expertise, and the department’s consulting firm, WSP.  Those three organizations plus the Community Task Force (CTF) chose ten criteria for evaluation:

  • Bridge Maintenance, Inspection, and Operations: What will this rehabilitate/replace concept need over its lifespan in terms of operations, maintenance, and inspection?
  • Constructability: the ease and efficiency with which each alternative can be built, relative to schedule and potential construction means and methods.
  • Environmental: What kind of temporary and permanent impacts will this rehabilitate/replace concept have on the Duwamish River and surrounding area? That includes waterway, public resources, emissions and wildlife impacts.
  • Equity: applying the city’s Racial Equity Toolkit to understand the disparate impacts the project might have on under-represented communities. Among other things, it looked at the duration of the bridge closure and the impacts on nearby underserved communities from rerouted traffic.
  • Forward Compatibility: Will this be compatible with Sound Transit light rail? Will it restore traffic capacity (weight and quantity) to the desired levels?
  • Funding Opportunities: What funding will be available, and what will the potential funding burdens be on local resources and communities? Is this rehabilitate/replace concept eligible for federal, state, local, or emergency funding?
  • Business and Workforce Impacts: impacts to vicinity businesses, including short- and long-term workforce impacts, based on the duration of closure, as well as additional construction impacts.
  • Mobility Impacts: the effects of construction and the bridge closure on regional mobility, including travel times, routes and mode shifts, as well as increased mobility once the crossing is reopened.
  • Multimodal Impacts: impacts to emergency vehicles, freight, and transit during construction, as well as impacts to pedestrians and bicycles in the final configuration.
  • Seismic/Safety: anticipated post-earthquake service and damage levels relative to the design seismic event, including consideration of implementation.

Here are the raw scores that the alternatives received:

But all the performance metrics are not treated equally: they are weighted, then combined to create a final score. Not surprisingly, there were some big differences in how the various groups wanted to weigh the ten criteria (and we need to be asking hard questions about why SDOT, the TAP, and the CTF had such divergent priorities). In the end their preferred weighting factors were averaged together — but we should definitely take heed that this is a major source of subjectivity in the report.

Here are the final, aggregate weighted performance scores for the alternatives, with Alternative 2 benchmarked to 500. Alternative 4 comes out on top, with 2 and 5 not far behind:


Next, the report looks at the cost estimates for each of the alternatives. There are two parts to this: the up-front capital costs to complete the immediate construction project and get traffic flowing again, and the “life cycle” costs for ongoing maintenance, additional anticipated repairs in future years, and potential demolition and replacement if before 2101.

Calculating this is tricky, as there are all sorts of hidden assumptions. One big one is whether to adjust for inflation of construction costs and/or the discount rate for future money. For the purposes of the main analysis, the report assumes a 0% discount rate, i.e. a dollar today is worth the same as a dollar in future years. That’s a provably false assumption, but it simplifies the calculation (and we’ll come back to this later, as they do provide a look at what happens if the discount rate is a more realistic figure — this is very important as some of the alternatives have huge up-front costs, while others have huge delayed costs).

First, the up-front capital costs. Cost of construction varies widely, from $47 million for Alternative 2 to add new steel tendons; to nearly $2 billion for Alternative 6 to build a tunnel. There are right-of-way acquisition costs in some cases, and some other variable costs. And then there’s “monetized risks”: to the extent that a price tag can be put on a major project risk, it’s added in. For example, Alternative 2, repairing the bridge, runs the risk of not being successful and requiring a replacement earlier than the hoped-for 40 years; it carries a $171 million monetized risk to account for the additional costs of replacement before 2062.

Second, there are the “life cycle” costs. These include operations and maintenance, repair and rehabilitation (or the cost of full replacement if before 2101, in the cases of Alternatives 1 and 2), and a credit for the “salvage” value of whatever useful life remains in the bridge in 2101. Putting the base capital costs and the life cycle costs together, we get the estimated cost over the entire life of each of the alternatives (the far-right column):

We mentioned the monetized risks in the capital costs. There are also monetized risks in the lifecycle costs as well, and we can break them out:

The report provides some useful details on the risks embedded in the cost estimates for the three viable alternatives (2, 4 and 5):

What you discover when you read carefully is that the top-line figures really sandbag Alternative 2, making the cost look much higher than it is likely to be by monetizing risks; both the initial capital costs, and the life cycle costs.  The risks are certainly real, but the details in the report make it clear that they are making fiscally conservative estimates — and since the recent bridge stabilization work has increased their knowledge, they know a lot more about the risks of repair than they do about the risks of the replacement options.


Also, the choice to use a 0% discount rate inflates the cost of Alternative 2 relative to the other options, since its upfront construction costs are minimal and the inevitable bridge replacement is 40 years down the line. Nevertheless, Alternative 2, from a cost point of view still looks cheap in comparison (though it would look less so if not for the $638 million salvage value credit).


The last step in the analysis is to make this a true “cost-benefit” analysis, putting the performance ratings and the costs together. Here’s the formula they use to arrive at a “value index” for each alternative — essentially the weighted performance score divided by the cost (a higher “value index” is better). Intuitively, this is “how much you’re getting for each dollar you spend.”

The report calculates the “value index” two ways: using just the upfront capital costs, and with the full life cycle cost. The weighted aggregate performance scores use Alternative 2 as a baseline of 500 (projects can be numerically higher or lower).

From a weighted performance score perspective, Alternative 4 comes out on top. But for the value index, Alternative 2 wins — and it especially kicks butt when looking at just the upfront capital costs, which is entirely to be expected since its near-term costs are by far the lowest. Imagine how much higher its scores would be if the cost calculations hadn’t incorporated so much monetized risk and had used a real discount rate.

Actually, we don’t have to imagine, because the report did a sensitivity analysis of several factors to see whether the results were tied to any specific assumptions.  Taking out the monetized risks, as expected, makes Alternative 2 look even better:

Using a discount rate of 3% doesn’t change the value indexes that are based solely on upfront capital costs (since it’s all essentially spent immediately), but the life cycle value indexes change (relatively) for the alternatives with deferred costs.

The report also recalculates the value indexes based on the original performance attribute weightings as specified by SDOT, the TAP, and WSP. Alternative 2 is the same because it’s still benchmarked to 500 for each of them, but you can see small relative changes in the others. In fact it’s a bit surprising that the different weightings didn’t make that much difference in the outcome (though it’s a great example of why it’s important to do sensitivity analyses).

There are a few other interesting tidbits in the report, one of which deals with the seismic rating of the bridge. The current West Seattle Bridge was categorized as “normal,” meaning that in a major earthquake it would be expected to remain intact but might need some repairs before it could be put back into service. If it were replaced, it would likely be re-categorized as “essential,” which would require it to be built to a standard such that it would remain in service immediately after a major earthquake. There were apparently concerns that the entire bridge only met the “normal” bridge standard, but the inspections over the last several months have convinced SDOT and its engineers and consultants that the basic structure of the bridge is in much better shape and (if the center span is repaired) meets the “essential” standard for bridge condition. So that is good news, and improves the prospects for Alternative 2 as a long-term solution.


So what’s the big takeaway from the report?  It’s clear that there are two alternatives that rise to the top: Alternative 2 (repair by adding new strengthening tendons), and Alternative 4 (replacing the superstructure). One requires a replacement in 40 years, the other in 50-75 years. Funding will be a challenge for all the options, but Alternative 2 requires the least up-front funding. All of the options require significant dedicated resources for ongoing operations and maintenance, however, and that’s something the city isn’t good at budgeting for.

One gets the sense reading the report that the writers perhaps threw some shade on Alternative 2 in order to avoid presenting a clear and obvious conclusion on whether to repair or replace the West Seattle Bridge — and thus evade a political fight. But a close read of the details leads to the inevitable conclusion that Alternative 2, repairing the bridge, is the best option. It’s the most affordable now, and most likely gives the city 40 years to plan and fund a full replacement. It gets traffic moving across the bridge two years from now, minimizing the mobility, workforce, environmental, and equity impacts of the current situation. The work is fairly well understood now, and assuming the fix to “un-stick” Pier 18 goes well in the next few weeks, there shouldn’t be major impediments. There are certainly some risks, including that the repair may not extend the bridge’s life for as long as is hoped, or that a new problem may be uncovered. But at this point there are no insurmountable obstacles to moving forward with repairing the bridge — and that’s a much-needed piece of good news.


I hope you found this article valuable. If you did, please take a moment to make a contribution to support my ongoing work. Thanks!



  1. I still don’t quite understand why they require an on-alignment replacement bridge under Alt 5 or 2 (40 years into the future). The current high bridge structure already jogs to the South and Spokane Street/the low bridge is the laser line. Plan to jog to the North with a replacement structure and just embrace that you might need to adjust some of the connection ramps when that happens and you can minimize the duration of any future closure related to a replacement bridge. Strategize some property options with the port to secure ability to use right of way in the future.

    At some point, SDOT should also suggest where in the lifespan the other structures are. If the Delridge ramps or Fauntleroy Expressway or the WSB ramp to 99N only have 40 years left it might be worth trying to see if those align with any anticipated project timeframes for the high bridge. (sometimes I really think a ramp to 99S would be nice).

  2. My issue with option 2 is how that will affect light rail introduction. Will we still be able to have light rail in West Seattle by 2030? What will be the costs of then replacing that 30 years after first introducing it?

    1. Alternative 2 wouldn’t have an option to incorporate light rail onto the bridge — but let’s remember that wasn’t on the table before the bridge broke either. Alternatives 4 and 5 would potentially allow that (though I’m skeptical because the parts of the bridge other than the main span aren’t getting replaced and they would likely end up being the constraint). But going that route would probably delay the project further to get in synch with Sound Transit and to align funding.

  3. Why is SDOT so intrigued by this massively expensive vanity project that WSP is promising –the rapid replacement new bridge option? Of course WSP will make all kinds of promises that they can get it done by 2023. But the Duwamish River is not anything like Lake Champlain. There is no reason to believe this will happen on schedule. We don’t need a $1B vanity project. We need a $47M fix. That’s so clear. The impact on our daily lives in WS is unsustainable. We need to repair the bridge! Thanks for this excellent analysis.

  4. This is an excellent analysis, especially the part about the zero percent discount rate they are using. For the life of me I can’t understand why we’re seriously considering anything other than option #2.

Comments are closed.