Seattle is very limited with the amount of land available. It's not feasible to build the 18 lane ultra freeway required to allow everyone to drive to their destination.

For better or worse Seattle city planners have taken a car-hostile approach to try and force people onto public transit. Unfortunately they also can't figure out how to pay for enough buses and trains to get everyone where they are going.

Also, my understanding is that there will be a surface street where the current viaduct is which combined with the tunnel will allow downtown access and increase capacity beyond what we have today.

So, planners there are car-hostile but are still spending billions to deliver as many total lanes as currently exist, counting both the tunnel and the surface street?

If that's the definition we're using, then I'd like someone to show hostility to an interest of mine by spending billions on it.

The comment I replied to was suggesting a second tunnel may need to be constructed to handle increasing traffic volume. My comment was pointing out that this is difficult for a number of reasons including the geographic reality of Seattle. The comment also assumed a car-first approach where roads have to meet increasing demand which is also not a priority in Seattle in 2017.

When the project is complete capacity will be the same or slightly larger than the capacity of a roadway built in the 1940s. Seattle has doubled in population since then. The current viaduct barely serves our needs and the new tunnel and surface street combo will not solve the transportation problems alone. We need fewer single occupant vehicles.

Cars are not given top priority in project planning. Just because one tunnel and one road are being put in place does not change that. There are transit corridors downtown that do not allow cars. Most downtown streets are a maze of one ways that make navigation difficult. Parking requirements for new construction are continually relaxed, reducing available parking across the city. Car tabs are expensive to fund for transit and discourage driving. Seattle has a lot of anti-car policies.

Planners here would like to spend more on building out our light rail and increasing bus/bicycle ridership than they would on adding subterranean exit ramps to a bypass tunnel.

e: Seattle's public transit system is largely bus-based so new road construction tends to take that into consideration with reserved lanes and stops and little attempt to serve the volume of cars.

Also, I don't think anyone claimed the tunnel project was supposed to solve traffic problems. The issue is that the viaduct itself is old and unstable. The tunnel is a combination of a safety improvement to a state highway and a subsidy for Olympia's favorite construction companies.

> Seattle is very limited with the amount of land available.

Tunnels don't use land.

The ramps to get in and out of them do though. Also tunnels do occupy space. Seattle already has a bus tunnel and a rail tunnel to avoid. On top of being built on Jell-O.

This is an important point, and one you can see in practice in downtown Boston walking above the I-93 tunnel - lots of land is still used for off-ramps and on-ramps, which tends to have high-speed traffic, whose presence is dangerous and discourages walking, which is the lifeblood of the city.

The exits, entrances, and ops building uses surface land.

There is limited subterranean land as well.

Adding lanes doesn't reduce traffic. It's called induced demand. In 1 year new capacity (supply) creates 50% in demand and in 5 years it is 100%. You want less traffic? Stop giving roads away for free.

Tunnels digging costs are roughly proportional to the square of their diameter, if we take Elon Musk's word for it.

So a six lane tunnel probably cost more than twice of four lanes.

> Tunnels digging costs are roughly proportional to the square of their diameter

Given a constant length and homogeneous ground composition (conditions that apply to the current discussion)... It makes a lot of sense that the cost is proportional to the volume being removed.

There has to be more to it than just homogenous ground composition. Surely drilling into granite is harder than clay or something? Also my understanding was that the SR99 tunnel is through both fill and clay.

http://www.wsdot.wa.gov/Projects/Viaduct/About/Tunneling

I may not have used the best phrasing:

For a given tunnel length and ground composition, the cost is proportional to the area of the cross section.

That's a great point, I failed to realize. This is a two-deck tunnel, so the price increase might not have been as steep.

In any event, some of the original proposals were for six lanes --wished they'd carried that out.

It was also already the world's largest tunnel boring machine.

Even a 4/3 multiplication of the diameter would have ~doubled the cross-section surface.

For what it's worth the project eliminates the downtown exits which is where a lot of people on the existing viaduct are either coming from or headed. Combined with a toll, I expect traffic over this section of roadway to improve. It's the other nearby roads that are concerning.

I see where they are just trying to alleviate things for thru traffic, but that leaves the side roads to make up for the traffic off-load --making it worse for the commute.

My complaint is that if you're going to invest a few billion into boring a tunnel, at least make it wide enough to carry all the orig traffic given that adding marginal width is a small fraction of overall cost of building a tunnel.

At 32 feet, it does seem like they have plenty of width for 3 lanes each way with no shoulder. 12 foot lanes are the standard for 60-70 mph interstates, but Aurora's top speed limit is like 45 mph or something and I think it's even lower in the current tunnel. So 10.6 foot lanes seem pretty reasonable.

Take a look at the sides of the I90 E tunnels. Tunnel walls tend to jump out at Seattle drivers. Gotta give people some breathing room and I'm sure there is some consideration for leaving space for emergency vehicles to squeeze by. I am also hoping they increase the speed limit inside the tunnel since there are no ramps. I think there is probably consideration for making it easy for buses to navigate as well.

I think you have a good point about emergency vehicle access.

I-90 is a much higher speed road, though. (Assuming they don't take your suggestion of increasing the speed limit in the tunnel.) Freeway standard is 12 feet per lane (and even the 1940-era Mt Baker tunnels have 12 foot lanes[0]), but again, Aurora isn't a freeway.

Meanwhile, buses are about 8.5 feet wide[1] and easily fit in a 10 foot lane. Most Seattle streets are narrower than that.

[0]: http://www.seattleasce.org/committees/hh/LaceyBridgeReport.p...

[1]: https://cptdb.ca/wiki/index.php/New_Flyer_Industries_XDE60

Maybe interesting datum: the existing Battery Street Tunnel has two lanes with a total width of 25 feet[0].

> The SR 99 roadway width narrows to 25 feet with two lanes in each direction through the BST

> The existing lane width varies from 10.5 feet to 11 feet, and there are no existing shoulders only a gore stripe adjacent to the curb... The existing posted speed is 40 mph.

So there's no real reason we couldn't keep the 10.5 foot lane size, 40 mph speed limit, and do three lanes. The new tunnel has better sight lines and curve radius than the BST. And there's separation between directions, which is better than the Aurora bridge (which also has pretty narrow lanes):

> Many aspects of the existing horizontal and vertical curves do not meet today’s roadway design standards for the posted speed limit. When compared to current design standards for stopping sight distance, horizontal curve radius, and vertical curve length, about two-thirds of the horizontal and vertical curves would coincide with a design speed of less than 40 mph.

[0]: http://www.scatnow.com/DesignDev3Grade.pdf

Is three lanes at 40mph higher throughput than two at 60?

Trick question! "mph" is not a direct measure of throughput. Cars per second (passing a given measurement point) is.

If the spacing of the cars is the same between the two, then a 60 mph lane is moving more cars per second than a 40 mph lane. The spacing never is the same, though, unless these are autonomous cars.

Also, throughput is limited by bottlenecks. If an upstream bottleneck has throttled things to one car every ten seconds, it doesn't matter whether the downstream lane is 30 mph or 60 mph.

Fine points but in this case posted MPH isn't an output, it's an input. So is the number of lanes.

Spacing is a function of speed. Even with autonomous cars spacing is not constant because there are on and off ramps so some will be accelerating or decelerating. We don't even know how those cars will work, maybe you can pay more to drive faster!

The question here is which configuration of the tunnel will maximize throughput.

I can't find any explicit statement of the planned speed limit but I did find a document [1] that implies there will be variable speed limits much like other highways in the area.

[1]: http://www.wsdot.wa.gov/Projects/Viaduct/Media/Default/-NewD...

Really good points. I was suggesting the need for a shoulder so people don’t just drive into the walls like they do on I90.

And what, put in a car elevator to get from the tunnel to first avenue? The tunnel is 200 feet underground at the location of the current ramps.

It's my understanding the first tunnel plans were for a six-lane tunnel. I have not seen the elevation/depth info on the tunnel, but even at 200f, you just need a longer ramp to ascend.

But you have to build that subterranean ramp. The tunnel cost billions already without the ramp, how much more would it cost with that added complexity? There will still be surface streets where the viaduct is located.

I'm not a transit engineer but I do think this is one of the drawbacks in the kind of radical democracy we push in Washington. Everything goes up to a public vote, even things the public can't really fully be experts on.