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
> 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.
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.
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...