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April 19, 2006

Rush Hour, By the Numbers

Sorry to be so Seattle-centric...but this post about Seattle's Alaskan Way Viaduct got me thinking. If the Viaduct is closed--whether for construction of a tunnel or a new aerial highway, or to make way for green space and a surface street--what happens to rush hour?  Does traffic in downtown Seattle get hopelessly snarled, and stay that way for at least 3 years?  Or do city transportation have some reasonable options for keeping people moving through the downtown core, even without a Viaduct?

Traffic studies show that the Viaduct carries about 105,000 daily trips. But most of those trips are at off-peak times when the surface streets have plenty of extra capacity. Sure, a trip along the Viaduct-less corridor would take a little longer than it does now; but the steet grid could easily handle the load.

But at rush hour -- particularly the afternoon -- there's precious little extra capacity on the city streets. So the thorniest problem that traffic planners will have to face will be accomodating rush hour trips on the street grid and I-5, during the busiest part of the day.

So, how many trips is that, exactly? And what are the options for dealing with the added load?

Earlier this week, the helpful and responsive folks at the Seattle Department of Transportation sent me some data that may help shed some light. As far as I can tell, it boils down to this: without the Viaduct, transportation planners will have to figure out how to accomodate the equivalent of 11,000 rush-hour car trips through the busiest part of downtown. Can they do it?

First, the data. (Feel free to skip the next few paragraphs if you're not a traffic geek.) Each year, the city collects data on traffic flows on the major arterials around Seattle, including the Viaduct and its on- and off-ramps. The data include figures for average weekday traffic, plus the traffic volume during the one-hour morning and afternoon peaks. Based on those figures, it seems like there are about 12,000 total vehicle trips on the Viaduct during busiest one-hour afternoon rush hour peak.

Now, without a Viaduct, some of those peak-hour trips will take quite a bit longer -- so people will shift their trips to other times of day, other modes, or forego them altogether. Based on published estimates of how much an increase in travel time decreases travel demand, it looks like demand for car trips may drop from 12,000 trips to 10,000 during the afternoon peak hour.

But some of those trips already begin or end in the Pioneer Square-to-Belltown corridor. A car that currently gets on at the Belltown northbound exit may have travelled through Belltown or downtown. So the peak number of cars added to surface streets after the Viaduct is closed will be somewhat less than 10,000.

To get a finer-grained look at where the traffic problems might be most severe, you can break down the trips on the former viaduct corridor into zones--Belltown, Downtown Core, Pioneer Square/Stadium, etc.--and look at the actual increase in travel demand in each zone. To me, it seems that the real pinch occurs in the Downtown Core--roughly, from Yesler in the south through Stewart in the north, and Alaskan Way on the west through Boren on the east. That area is already pretty packed during rush hour. With no Viaduct, peak-hour travel demand will increase by somewhere between 6,100 and 7,200 trips. (Note, this is a somewhat conservative estimate -- I'm assuming that some former Viaduct trips will "disappear"--i.e., move to other times or destinations--because they'll take too long; but I'm not assuming the same for trips already on the surface streets.)

So that's the one-hour peak. Over the course of a rush hour that lasts at least an hour and a half, that means that transportation officials will have to worry about accomodating the demand for some 11,000 addtional trips in the busiest part of downtown, during the busiest part of the day.

So, 11,000 extra trips: is that a lot or a little? It's a lot less than 105,000. But in my mind, it's still a lot of trips. The existing street grid may be able to hold a few more cars than it currently does. Some tweaks to traffic enforcement ("don't block the box"), elimination of some street parking during rush hour, and so forth may increase throughput a bit more. Still, even with those improvements, the demand for 11,000 extra trips could really jam up the afternoon rush hour. Even if people eventually adjust to the congestion -- by changing schedules or jobs, or switching to transit -- the early months could be brutal.

But if you add in transit improvements, accomodating 11,000 downtown trips seems much more achievable. The bus system already carries 31,000 people out of downtown during the afternoon peak. So getting 11,000 people to shift from driving alone to the bus would boost rush-hour transit ridership by a little more than a third -- tough, maybe, but not inconceivable.

And in theory, at least, there's ample capacity to handle that many trips in the bus tunnel, which is now closed for service. Once the tunnel reopens, it will be able to handle about 9,000 rush hour trips that right now are travelling on Third Ave. And when light rail starts running through the tunnel, its capacity could grow by a third or more. (To my surprise, it seems that the tunnel may have been underutilized; one estimate, a few years old now, is that the tunnel could carry 18,000 trips per hour (scroll down a bit to find the claim), with everyone seated, in buses alone. If that's really possible, then the tunnel alone would be meet the post-Viaduct demand.)

And then there's Third Ave., which is currently closed to cars during rush hour. If Third reverts to being predominantly a car corridor, it'll handle at most 2,000 vehicle trips during rush hour. But if it's kept closed to cars, and is used to handle an extended bus schedule, it can handle at least three times as many passengers.

So, there are three options -- (1) surface street and traffic enforcement tweaks, (2) adding light rail to the bus tunnel, and (3) keeping Third Ave. as a bus-only street even after the bus tunnel opens -- that could accomodate most, if not all, of the added demand for rush hour trips. If those options are phased in, as the Viaduct is phased out of service, it could be that many folks wouldn't notice much of a change to their afternoon commutes.

Yes, it would be tough to get people out of their cars onto transit. But if city officials have their way -- and the Viaduct is closed for reconstruction -- they won't have much choice but to try. There really aren't many other options.

And, as I've said before -- if a combination of transit and street improvements can keep downtown traffic moving, or at least bearable, for a mimimum of three years, why not see if they'll work as a permanent solution? Why spend billions of dollars to fix a problem that the city's already solved?

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Why do you buy apparently into the myth that the Viaduct cannot be repaired/retrofitted while in use? Have you read any studies which document the myth??

Posted by: David Sucher | Apr 19, 2006 11:56:34 PM

Fair question, David. There are 3 reasons I hesitate to talk about the retrofit option too much:

1) I talked a few months ago with an engineer who specialized in studying how earthquakes affect soils and physical infrastructure. His view (as I recall the conversation) was that stiffening the Viaduct at vulnerable points has at least two flaws: it doesn't address soil liquefaction, which can still cause catastrophic failure even in the repaired portion of the viaduct; and that stiffening the viaduct in one place can increase the chance of failure elsewhere on the structure.

He said he'd looked at the Twelker/Gray proposal, and didn't think it worked. In his view, a repaired viaduct could still fail catastrophically with a Nisqually-style earthquake with the epicenter closer to Seattle. This made sense to me, and it pretty much agrees with the state's studies.

2) I'm a wimp, or maybe a sensitve new age guy, or somesuch. At one point, I was personally angling towards the view that the standard risk assessment -- that the Viaduct faced a one-in-twenty chance of collapsing in the next decade -- suggested that fixing the Viaduct really wasn't worth the cost. I mean, in a serious earthquake there are plenty of other equally large risks, and probably some greater ones: other bridges that might collapse, buildings that need earthquake retrofits, etc. Both from a public safety and a transportation point of view, it seemed like > $3 billion to improve the safety of 2 miles of road wasn't getting much bang for the buck. In fact, the most "efficient" thing to do would probably be to leave the viaduct as is, and do something else with the money.

(This is partially informed by the experience of my sister-in-law, who was pinned under a collapsed beam in the Loma Prieta earthquake -- the one that brought down the Cypress St. Viaduct, killing 42. Rescuers cut her out with a chain saw; she escaped with minor injuries, but other people around her weren't so lucky.)

But then, I imagined myself facing a room full of parents, old and young, whose kids had died in a Viaduct collapse. How would I feel, explaining my opposition to making the Viaduct as safe as possible? Not good at all, I realized. And if I wasn't willing stand up in public afterwards and say "I'm sorry that your kids died, but hey, I still think it was the right decision," then perhaps I'm really much more risk averse about these sorts of things than I thought I was.

(I'm a new parent, and the thought of a parent losing a child--at whatever age--still brings a lump to my throat.)

3) I'm not inherently opposed to studying the idea, really. But I think there's a risk that you can study anything until you find the answer you want -- and then use the one study to call into question all the others.

That's the dynamic for global warming: a handful of studies that suggest it's not happening are giving cover for policies that are contrary to the overwhelming scientific consensus.

Not being able to judge the issues for myself, I have to rely on the collective judgment of experts. And my perception is that most other seismologists & structural engineers who have studied a retrofit simply disagree with Twelker and Gray. There may be politics involved there, of course, or a failure of creativity.

But my engineer friend's response convinced me that any pro-retrofit study would be quickly countered by anti-retrofit studies; that the public would soon get confused about who's right; and that we'd be in the same place we are now: making an expensive choice in a climate of uncertainty, guided more than anything else by what we, collectively, *want* to be true.

Posted by: Clark Williams-Derry | Apr 20, 2006 10:36:27 AM

Sorry for muddying the waters further, but I think there's a fourth option for the viaduct that I haven't heard discussed anywhere (the other three being tunnel, viaduct, surface). Why can't we build a covered ground-level road? Then level the grade on the east side, slope it on the west side, and build streets, sidewalks, and even parks over it. Effectively raise the ground level in this area. This should be much cheaper than all of the seawall/digging costs of a tunnel, seperate long-distance traffic from local traffic, and preserve the land from view-stealing commercial developments.

It's just a thought that's been bugging me for a while now. In regard to this issue, it would be possible to have it open very quick since you could more or less construct it before you tear down the viaduct.

Posted by: Matt | Apr 20, 2006 10:52:53 AM

Matt -

My first impression is that this is, in essense, a lot like the cut-and-cover tunnel option -- only instead of excavating & carting away some soil to make the tunnel, you're excavating some soil somewhere else, carting it in, and putting it next to the tunnel. I don't have any idea if that saves money vs. the tunnel option, but it might.

Posted by: Clark Williams-Derry | Apr 20, 2006 11:44:20 AM

Part of the problem, Clark, is that leadership on this issue has been so poor that FIVE years after the earthquake we are still far from a plan. Part of that poor leadership has been to do exactly what you are suggesting isn't right:

"....study anything until you find the answer you want -- and then use the one study to call into question all the others."

I believe that the tunnel answer to the viaduct issue is a perfect example and corrodes public trust -- at least my trust -- in the answers we've seen so far.

I have in fact read the first studies on the seawall -- I blogged on it two years ago -- and they were not very persuasive, as well as being suspiciously convenient when trying to gin up support for a big public work.

No, I don't know who you have talked to but if they truly believe it then they should come out front and say it in public, and with facts.

Anyway, the money isn't there for a tunnel and the interim transport issue is the deal killer.

Posted by: David Sucher | Apr 20, 2006 11:48:01 AM

As to Matt's plan, would that make what is now first-floor retail along Western into basement space?

(I am trying to visualize the end result.)

Posted by: David Sucher | Apr 20, 2006 11:52:45 AM

Excellent analysis, Clark. Especially because it reveals the wimpy logic behind the "we have a highway now so we must have a highway in the future" assertion by the state DOT.

To paraphrase your thinking: trips come from and go to many destinations, and there is capacity in the grid and potential capacity on new transit, so let's try to match demand with supply that is a lot cheaper and easier to get. That is just the kind of unglamorous, practical and sensible problem-solving that we need more of on this project.

Your suggestions for how to shift trips to transit are solid, nothing to add there. I think there are more tools in the toolbox you didn't mention: demand management and traffic engineering to improve functionality of the street grid.

Your readers, of course, can cite all the popular demand management strategies these days: tolling, congestion pricing, parking cash out, matching employees to work sites closer to where they live, parking taxes, flexible work schedules, transit pass subsidies, etc. All relatively cheap and politically challenging, but some mix could be highly effective in this situation.

To get more flow thru existing streets, the City's Central City Access Strategy named many many specific operational improvements. Making more streets cross Denny so there are more north / south options; fixing bottleneck intersections; better distributing trips south of downtown onto Alaskan Way, 4th, 6th, and Airport Way. Better link the central part of Alaskan Way into the grid. Computerize control of lights to get 10% to 15% more flow on existing pavement.

With all these improvements, common sense says you could probably absorb 1/2 the peak hour trips onto 5 or 6 parallel streets, and the rest can move to transit, be shifted to other times of day, or not happen.

Posted by: Cary | Apr 20, 2006 12:17:30 PM

Clark -

Yes, it is like the tunnel option, except it solves a few issues:
1) A tunnel would be below water level, bringing it's own set of problems (sea wall, drainage)
2) We don't seem to be good at tunnels in the northwest. What we are good at is building lids.

Of course you also free the project up from digging into the the ground, which has it's own set of issues and costs (re-routing piping, storm water, etc.).

David -

More or less correct. Right now there's a steep grade on the north side, so it won't be much of an issue. On the south side I am envisioning some significant grade changes - either by changing the 1st floor of some buildings to basement level or by the less attractive option of having a small hill (downward, up a bit, back down as you approach the water).

Posted by: Matt | Apr 20, 2006 3:15:01 PM

A few comments:


The I-90 tunnel is one the best example of a successful tunnel project in the world, so I'm not sure how WSDOT deserves the "can't do tunnels" label. A cut and cover "tunnel" isn't really a tunnel anyway. It's a hole with a lid on it. Utilities would have to be resited for any project.

WSDOT did consider an encased roadway as part of the long-list prior to the short list of 5. I believe their conclusion was that it would cost about the same as a tunnel with fewer benefits. WSDOT would probably give a general answer in short order if you asked them.


With regards to Twelker's "conservative surgey idea" the onus is on them to prove that a) only one section of the viaduct requires retrofit and b) that the retrofit wouldn't simply redistribute the stresses to the foundation or another section. Putting a band-aid on the one section doesn't bring the rest of the structure up to current seismic code. You'd need to do a full retrofit to do that, which WSDOT says would cost only a bit less than a rebuild. WSDOT is basically not willing to gamble on the structure's seismic safety and for that objective I can't fault them. I can however fault them for not working to mitigate traffic impacts now so we can shut the thing down asap.

Posted by: smiles | Apr 20, 2006 4:30:32 PM

"...the onus is on them to prove..."

I think not.

If that were sound politics then it should be on Cary Moon's group to prove, without benefit of governmental money to develop a sophisticated plan, to "prove" that the tear-down approach works.

In equity and practical politics, citizen efforts like Cary Moon's and the Twelker/Gray group must have their ideas examined and elaborated fairly and deeply with government backing.

It doesn't make sense to me to say that the onus is on Twelker to "prove" anything at all. Gray & Twelker are highly-regarded engineers. If you accept Cary's wishful thinking (sorry Cary) as a basis for substantial commitmkent of govt study then it's absurd to deny that to guys who have many many decades of experience as engineers.

Posted by: David Sucher | Apr 20, 2006 9:08:00 PM


There is a small difference between Twelker and Gray's idea and that of the PWC: the former has been studied and reviewed extensively by WSDOT engineers and many other equally respected engineers and the latter hasn't. The onus is on Twelker and Gray to show that these previous studies got it wrong and their idea doesn't put lives at risk. The onus is on PWC equally to prove to folks that it's worth putting resources equal to the retrofit studies into studying their idea, which may annoy some folks with a bit of extra commute time (maybe not) but won't put any lives in danger. PWC is close to making their case to get a similar study. If a study is done, show's PWC is wrong...then they'll be in the same spot as Twelker and Gray.

Two studies to look at on the possibility of retrofitting the viaduct and its structural deficiencies with respect to seismic code are :



Posted by: smiles | Apr 20, 2006 10:58:14 PM

Some basic comments - The concept that car trips can disappear, people will hop on buses, traffic enforcement can be ramped up, that this is all a matter of just designing away a solution doesn't deal with the reality that a lot of people, those that use the Viaduct, and those that don't, are going to have a good portion of their lives shredded by all of this. These social engineering schemes fail to consider the impact on people. Cars, "passengers on buses", "trips" are people, they aren't inanimate objects that can just be shuffled around like so many game pieces, part of some great, strategic engineering move. How are we providing for the stress and pain that the people impacted by this project are going to experience? Giving them tickets, denying them the opportunity to freely move in areas where they once could move unfettered, and forcing them to associate with a lot of people that they formerly didn't want to associate with (on buses) seems like hardly a "solution".

There is also quite a bit of preconceptions and presumptions in these traffic plans. One thing that can't be planned for is how people will really react to a radical change in their traffic patterns. Just like water, traffic seeks its own level, and all the planning in the world will not be able to control that. These traffic plans rely on static conditions. What would the effect be for example of a few more office or apartment building construction projects taking place in the area at the same time? We all know how they like to sprawl out into the streets, and how their deliveries and workers clog up the streets around the project. This is very likely going to happen; have we forgotten the new building heights limits - some of those projects will materialize just as the grand traffic plan is being put into effect. And really, the building projects are given priority everytime over traffic, whether its bus, car, or even business related traffic - trucks. Which brings up another point, the "traffic" includes quite a bit of business related travel. That segment isn't likely to avail itself of bus solutions or traffic obstruction schemes.

In addition, there is this sense that all these changes are temporary, that we'll just reroute things for awhile and then once whatever Viaduct solution is completed things will go back to normal. This failing again to account for the human factor. Rampant traffic can very likely negatively impact Downtown, Pioneer Square, and the Waterfront for some years, both during and after construction. Tourists, workers, and business people that go away during this time may not so easily be persuaded to come back. There may be a residual effect for years to come.

There is more to come, including a third construction alternative - a landmark bridge solution - not in Elliott Bay, but in the Viaduct corridor. I'll go through it later, but this is a real compromise that the Cary Moons and the Twelkers of the world can perhaps wrap their visions around. They and everyone else, like the Mayor and WSDOT, need to remember that it is all about compromise.

Posted by: Elizabeth | Apr 21, 2006 4:14:09 AM

"There is a small difference between Twelker and Gray's idea and that of the PWC: the former has been studied and reviewed extensively by WSDOT engineers and many other equally respected engineers and the latter hasn't."

So? (Assuming you are accurate, which I don't believe is so.) Have WSDOT and outside consulting engineers review it.

Your resistance (and others) to studying retrofit approaches is just so puzzling. My conclusion is that you and others do NOT want the possibility of a retrofit...otherwise why the resistance?

I personally don't think that the PWC's plan will work. But I support 100% -- read my blog -- giving it a fair study. Why are you afraid of honestly studying the retrofit approach?

Posted by: David Sucher | Apr 21, 2006 8:35:32 AM


What I'm hearing about the retrofit is not that anyone's opposed to having it studied - rather, it's *been* studied and found wanting. Given the erstwhile cost savings, why would WSDOT not seize on the chance to save those billions and pour them into some other freeway project?

Even Nick Licata, who flew the retrofit flag for a long time, doesn't speak about it much anymore. Seems like there's a lot more to talk about in Clark's analysis that's worth more discussion than bickering about a retrofit.

Posted by: John | Apr 21, 2006 11:14:53 AM

If it has been found wanting (by people who have a stake in a bigger/different project) then why are Twelker and Gray suggesting it? Do you think they are fools? If nothing else they have their professional reputations at stake.

It's so amazing to me that so many people who are otherwise suspicious of WSDOT are so willing to suspend their skepticism and believe as a matter of faith that the existing viaduct cannot be retrofitted. The psychology of this issue is fascinating. I can speak from personal experience that it is extremely difficult to engage cerrtain of the PWC leaders in any discussion of the retrofit. They airily wave it off by quoting unnamed engineers and WSDOT "studies" -- the same WSDOT with whome they otherwise don't agree.

Now let's be clear -- I am not an engineer and I make NO claim that a retrofit will work. What fascinates me is the politics -- the refusal to consider a retrofit on the part of the Mayor's office, WSDOT, the PWC, Allied Arts etc etc...Even though these folks disagree about the solution they are united in opposition to a serious consideration of a retrofit. Of course that's what I think we will actually get..the Retrofit..which is one of those delicious ironies.

It's fascinating.

Posted by: David Sucher | Apr 21, 2006 12:07:43 PM

It is interesting, David.

I think that part of the reason that people don't view the retrofit as viable is that, as far as I know, everyone other than Twelker/Gray who's looked seriously at the issue has concluded that you can't just fix one part of the viaduct and have it meet modern seismic standards & codes.

For example, the 1995 paper smiles references above talks about soil liquefaction risks in fairly stark terms. Two choice quotes:

"The manner in which the waterfront fills beneath the Alaskan Way Viaduct were placed is a virtual recipe for creating a liquefiable soil deposit."

"Widespread liquefaction is expected to occur in a design-level ground motion and
could cause multiple sections of the viaduct to collapse."

This from UW engineering professors, peer reviewed by colleagues at CA universities with specialties in seismic engineering. And from the other paper:

"The evaluations of the existing Viaduct, coupled with assessments of damage after
the Nisqually event, place the threshold earthquake event for collapse of major
portions of the Viaduct at a return period of 210 years."

I'm not sure what "major portions" means, exactly, but it sounds like the problem isn't just one section. Perhaps someone should ask the members of the committee that produced the document.

Now, one reason *not* to believe those studies is that they've all been funded by WashDOT, which might have its own agenda. But as far as I can tell, these are the studies that set the stage for the original tunnel/rebuild/retrofit debate -- the earlist that I know of (1995) was completed before the tunnel idea was even on the drawing board. They seem to me like a good faith effort to assess the stability of the viaduct, not to add fuel for the fire once the decision to build had already been made.

Maybe the issue, then, is this: what is the public to do when experts simply disagree? One option is to keep studying the issue from every possible angle, until you reach a firm conclusion that virtually everyone agrees with. Another is to wait until a sufficient body of evidence and opinion emerges; weigh the relative numbers, qualifications, and motivations of the various experts; review what they have to say; and make the best judgment you can about whose arguments are best supported, factoring in all of the above.

My perception is that the short shrift given to Twelker's idea is based on the latter kind of reasoning: a bunch of qualified experts have weighed in, staking their professional reputations on their analyses; and the large majority think that you can't do a cheap retrofit and have the structure even come close to meeting code.

Obviously, there's some question about whether "code" is the right standard for safety -- how many lives do you save with a $4 billion dollar tunnel vs. a $200 M retrofit? What else could you do with that money? But as whether a scaled-back retrofit would meet today's codes, my view is that the matter is fairly well settled.

Posted by: Clark Williams-Derry | Apr 21, 2006 1:46:52 PM

None of your quotes is on point.
If liquifaction is a problem then it is a problem for all solutions (except PWC's).
A NEW viaduct would also have to contend with it in some fashion. So would a tunnel.
I don't see any new information in what you are offering.

Posted by: David Sucher | Apr 21, 2006 4:10:50 PM

No, David, preventing soil liquefaction is part of *all* three options that WashDOT/SDOT studied seriously. The plan was to use jet/compaction grouting and some other soil stabilization techniques. It's expensive -- and (I believe) one of the reasons that even the retrofit was so costly.

See here for WashDOT's basic answer.

"What about tunnels built in poor quality, liquefiable soil, like along the Seattle waterfront?"

"The Viaduct project team is taking the liquefiable soil beneath the existing structure into account, and the project will include extensive soil strengthening to ensure the future structure's stability in an earthquake.

The Viaduct was originally built on loose fill, soft sediment, sand, and gravel. Much of this will be removed during tunnel excavations. Additionally, in the south end of the project area where SR 99 will be at ground level, below-ground soil improvements will still be required for on and off ramp bridges. Soils can be strengthened by using jet grouting or deep soil mixing techniques. These techniques inject, mix, or replace the existing soil with cement grout. Furthermore, both of the foundations for the Tunnel or Elevated Structure will be anchored in either the strengthened soil or dense glacial soils below the surface."

See more on liquefaction at this UW civil engineering website. http://www.ce.washington.edu/~liquefaction/html/main.html

See you over at http://sightline.org !

Posted by: Clark Williams-Derry | Apr 24, 2006 4:03:51 PM

Nice smackdown, Clark! Case closed. Retrofit = deathtrap

Posted by: citydweller | Apr 25, 2006 12:33:59 PM