The question of which buses should be in the tunnel has been asked in at least two different ways. Right now, the buses that are slated to remain are the 41, 74, 101, 102, 106, 150, 255. I agree with David Lawson, that the 255 should be removed. I think it should be replaced by either the 77 or the combination of the 76 and 316.
One of the big advantages to the bus tunnel is that it feeds very well into the express lanes. The 255 does not use the express lanes, so it does not gain much of an advantage (if it gains one at all) by using the bus tunnel.
It also makes sense to cluster similar bus routes into the tunnel, or out of the tunnel. The old 71,72, 73 and 74, for example, all served the U-District, and they all went in the tunnel. This meant that someone headed to the U-District from downtown simply waited for the first available bus starting with a “7”. The 255 doesn’t pair well with any other bus that travels in the tunnel. It is possible that someone might want to head to Montlake on either the 255 or Link, but I doubt very many.
The buses that use (or could use) the express lanes, but are not slated to use the tunnel are the 76, 77 and 316. By my estimation, they make the following number of trips:
76 — 11 trips at rush hour.
77 — 8 trips at rush hour.
316 — 11 trips at rush hour.
By removing the 255, we would remove 23 trips at rush hour. So, obviously, we can’t put all three in the tunnel. Each bus overlaps (or shares) some of the other routes in the tunnel, so there are several possibilities. These are the two that I believe make the most sense:
76 and 316 — They both serve the Green Lake Park and Ride. They diverge from there, but since there are a lot of transfers from there, and this a fairly populous area, the shared ridership is probably significant. In addition, the 76 and 74 are close enough in service area to have significant overlap (for those who live in between the routes).
77 — The 77 and 41 both serve Pinehurst (NE 125th and 15th NE). This also means either one works as the first bus to the 347/348. In addition, those headed to the area served by the 77 can take a 74 (or Link) and then transfer to the 73 or 373 instead. Replacing the 255 with the 77 would mean significantly fewer buses in the tunnel. This could improve reliability in the tunnel, especially for Link.
Either option (either the 77 or the combination of the 76 and 316) would be an improvement over the 255. The 255 does not take advantage of one of the best features of the bus tunnel (its connection with the express lanes) nor does it’s service area overlap with buses that will be in the tunnel.